Feedback method of hybrid csi in multi-antenna communication system and device therefor

ABSTRACT

Disclosed in the present invention is a method by which a terminal reports channel status information (CSI) to a base station in a wireless communication system. The method comprises the steps of: receiving, through an upper layer, information on one CSI process having a first enhanced multiple input multiple output (eMIMO) type and a second eMIMO type; receiving a first channel status information-reference signal (CSI-RS) corresponding to the first eMIMO type; periodically reporting, to the base station, first CSI measured on the basis of the first CSI-RS; receiving a second CSI-RS, which corresponds to the second eMIMO type, beamformed on the basis of the first CSI; and periodically reporting, to the base station, second CSI measured on the basis of the second CSI-RS, wherein a priority of the first CSI is set to be equal to a priority of a CSI-RS resource indicator (CRI).

TECHNICAL FIELD

The present invention relates to a wireless communication system, andmore particularly, to a method and apparatus for feeding back hybridchannel status information (CSI) in a multi-antenna communicationsystem.

BACKGROUND ART

3GPP LTE (3rd generation partnership project long term evolutionhereinafter abbreviated LTE) communication system is schematicallyexplained as an example of a wireless communication system to which thepresent invention is applicable.

FIG. 1 is a schematic diagram of E-UMTS network structure as one exampleof a wireless communication system. E-UMTS (evolved universal mobiletelecommunications system) is a system evolved from a conventional UMTS(universal mobile telecommunications system). Currently, basicstandardization works for the E-UMTS are in progress by 3GPP. E-UMTS iscalled LTE system in general. Detailed contents for the technicalspecifications of UMTS and E-UMTS refers to release 7 and release 8 of“3rd generation partnership project; technical specification group radioaccess network”, respectively.

Referring to FIG. 1, E-UMTS includes a user equipment (UE), an eNode B(eNB), and an access gateway (hereinafter abbreviated AG) connected toan external network in a manner of being situated at the end of anetwork (E-UTRAN). The eNode B may be able to simultaneously transmitmulti data streams for a broadcast service, a multicast service and/or aunicast service.

One eNode B contains at least one cell. The cell provides a downlinktransmission service or an uplink transmission service to a plurality ofuser equipments by being set to one of 1.25 MHz, 2.5 MHz, 5 MHz, 10 MHz,15 MHz, and 20 MHz of bandwidths. Different cells can be configured toprovide corresponding bandwidths, respectively. An eNode B controls datatransmissions/receptions to/from a plurality of the user equipments. Fora downlink (hereinafter abbreviated DL) data, the eNode B informs acorresponding user equipment of time/frequency region on which data istransmitted, coding, data size, HARQ (hybrid automatic repeat andrequest) related information and the like by transmitting DL schedulinginformation. And, for an uplink (hereinafter abbreviated UL) data, theeNode B informs a corresponding user equipment of time/frequency regionusable by the corresponding user equipment, coding, data size,HARQ-related information and the like by transmitting UL schedulinginformation to the corresponding user equipment. Interfaces foruser-traffic transmission or control traffic transmission may be usedbetween eNode Bs. A core network (CN) consists of an AG (access gateway)and a network node for user registration of a user equipment and thelike. The AG manages a mobility of the user equipment by a unit of TA(tracking area) consisting of a plurality of cells.

Wireless communication technologies have been developed up to LTE basedon WCDMA. Yet, the ongoing demands and expectations of users and serviceproviders are consistently increasing. Moreover, since different kindsof radio access technologies are continuously developed, a newtechnological evolution is required to have a future competitiveness.Cost reduction per bit, service availability increase, flexiblefrequency band use, simple structure/open interface and reasonable powerconsumption of user equipment and the like are required for the futurecompetitiveness.

DISCLOSURE Technical Problem

A method and apparatus for feeding back hybrid channel statusinformation (CSI) in a multi-antenna communication system are proposedbelow on the basis of the above discussion.

It will be appreciated by persons skilled in the art that that theeffects that can be achieved through the present disclosure are notlimited to what has been particularly described hereinabove and otheradvantages of the present disclosure will be more clearly understoodfrom the following detailed description.

Technical Solution

According to an embodiment of the present invention, a method ofreporting channel status information (CSI) to a base station by aterminal in a wireless communication system includes receivinginformation about one CSI process including a first enhanced multipleinput multiple output (eMIMO) type and a second eMIMO type through ahigher layer, receiving a first channel status information-referencesignal (CSI-RS) corresponding to the first eMIMO type, periodicallyreporting first CSI measured based on the first CSI-RS to the basestation, receiving a second CSI-RS corresponding to the second eMIMOtype, beamformed based on the first CSI, and periodically reportingsecond CSI measured based on the second CSI-RS to the base station. Apriority of the first CSI is set to be equal to a priority of a CSI-RSresource indicator (CRI).

Meanwhile, according to an embodiment of the present invention, aterminal in a wireless communication system includes a wirelesscommunication module configured to transmit and receive signals to andfrom a base station, and a processor configured to process the signals.The processor is configured to receive information about one CSI processincluding a first eMIMO type and a second eMIMO type through a higherlayer, to periodically report, to the base station, first CSI measuredbased on a first CSI-RS corresponding to the first eMIMO type, and toperiodically report second CSI measured based on a second CSI-RScorresponding to the second eMIMO type, beamformed based on the firstCSI. A priority of the first CSI is set to be equal to a priority of aCRI.

Specifically, if the first CSI including a precoding matrix index (PMI)collides with the second CSI including a rank indicator (RI), the secondCSI may be dropped.

Preferably, a reporting period of the first CSI may be determined to bea multiple of a longest reporting period of the second CSI. Herein, thelongest reporting period of the second CSI may be a reporting period ofan RI included in the second CSI. Further, if the RI is not included inthe second CSI, the longest reporting period of the second CSI may be areporting period of a channel quality indicator (CQI) included in thesecond CSI.

More preferably, an offset for the reporting period of the first CSI maybe determined based on an offset for the longest reporting period of thesecond CSI.

Advantageous Effects

According to the embodiments of the present invention, feedback ofhybrid channel status information (CSI) can be carried out moreeffectively in a multi-antenna communication system.

It will be appreciated by persons skilled in the art that the effectsthat can be achieved with the present disclosure are not limited to whathas been particularly described hereinabove and other advantages of thepresent disclosure will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the disclosure and are incorporated in and constitute apart of this application, illustrate embodiments of the disclosure andtogether with the description serve to explain the principle of thedisclosure. In the drawings:

FIG. 1 illustrates a configuration of an evolved universal mobiletelecommunications system (E-UMTS) network as an example of a wirelesscommunication system;

FIG. 2 illustrates a control-plane protocol stack and a user-planeprotocol stack in a radio interface protocol architecture conforming toa 3rd generation partnership project (3GPP) radio access networkstandard between a user equipment (UE) and an evolved UMTS terrestrialradio access network (E-UTRAN);

FIG. 3 illustrates physical channels and a general signal transmissionmethod using the physical channels in a 3GPP system;

FIG. 4 illustrates a structure of a radio frame in a long term evolution(LTE) system;

FIG. 5 illustrates a structure of a downlink radio frame in the LTEsystem;

FIG. 6 illustrates a structure of an uplink subframe in the LTE system;

FIG. 7 is an exemplary view illustrating the configuration of a generalmultiple input multiple output (MIMO) communication system;

FIG. 8 is an exemplary view illustrating coordinated multi-point (CoMP)implementation;

FIG. 9 is a view illustrating a downlink CoMP operation;

FIG. 10 is a view illustrating an implementation example of atwo-dimensional active antenna system (2D-ASS);

FIG. 11 is an exemplary view illustrating the concept of hybrid channelstatus information (CSI);

FIG. 12 is an exemplary view illustrating a CSI relaxation method forhybrid CSI according to an embodiment of the present invention;

FIG. 13 is an exemplary view illustrating another CSI relaxation methodfor hybrid CSI according to an embodiment of the present invention;

FIG. 14 is an exemplary view illustrating hybrid CSI reporting accordingto an embodiment of the present invention;

FIG. 15 is another exemplary view illustrating hybrid CSI reportingaccording to an embodiment of the present invention; and

FIG. 16 is a block diagram of a base station (BS) and a UE, which areapplicable to an embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The configuration, operation, and other features of the presentinvention will readily be understood with embodiments of the presentinvention described with reference to the attached drawings. Embodimentsof the present invention as set forth herein are examples in which thetechnical features of the present invention are applied to a 3rdGeneration Partnership Project (3GPP) system.

While embodiments of the present invention are described in the contextof Long Term Evolution (LTE) and LTE-Advanced (LTE-A) systems, they arepurely exemplary. Therefore, the embodiments of the present inventionare applicable to any other communication system as long as the abovedefinitions are valid for the communication system. In addition, whilethe embodiments of the present invention are described in the context ofFrequency Division Duplexing (FDD), they are also readily applicable toHalf-FDD (H-FDD) or Time Division Duplexing (TDD) with somemodifications.

FIG. 2 illustrates control-plane and user-plane protocol stacks in aradio interface protocol architecture conforming to a 3GPP wirelessaccess network standard between a User Equipment (UE) and an EvolvedUMTS Terrestrial Radio Access Network (E-UTRAN). The control plane is apath in which the UE and the E-UTRAN transmit control messages to managecalls, and the user plane is a path in which data generated from anapplication layer, for example, voice data or Internet packet data istransmitted.

A PHYsical (PHY) layer at Layer 1 (L1) provides information transferservice to its higher layer, a Medium Access Control (MAC) layer. ThePHY layer is connected to the MAC layer via transport channels. Thetransport channels deliver data between the MAC layer and the PHY layer.Data is transmitted on physical channels between the PHY layers of atransmitter and a receiver. The physical channels use time and frequencyas radio resources. Specifically, the physical channels are modulated inOrthogonal Frequency Division Multiple Access (OFDMA) for Downlink (DL)and in Single Carrier Frequency Division Multiple Access (SC-FDMA) forUplink (UL).

The MAC layer at Layer 2 (L2) provides service to its higher layer, aRadio Link Control (RLC) layer via logical channels. The RLC layer at L2supports reliable data transmission. RLC functionality may beimplemented in a function block of the MAC layer. A Packet DataConvergence Protocol (PDCP) layer at L2 performs header compression toreduce the amount of unnecessary control information and thusefficiently transmit Internet Protocol (IP) packets such as IP version 4(IPv4) or IP version 6 (IPv6) packets via an air interface having anarrow bandwidth.

A packet data convergence protocol (PDCP) layer at L2 performs a headercompression function to reduce unnecessary control information forefficient transmission of an Internet protocol (IP) packet such as anIPv4 or IPv6 packet in a radio interface having a relatively narrowbandwidth.

A Radio Resource Control (RRC) layer at the lowest part of Layer 3 (orL3) is defined only on the control plane. The RRC layer controls logicalchannels, transport channels, and physical channels in relation toconfiguration, reconfiguration, and release of radio bearers. A radiobearer refers to a service provided at L2, for data transmission betweenthe UE and the E-UTRAN. For this purpose, the RRC layers of the UE andthe E-UTRAN exchange RRC messages with each other. If an RRC connectionis established between the UE and the E-UTRAN, the UE is in RRCConnected mode and otherwise, the UE is in RRC Idle mode. A Non-AccessStratum (NAS) layer above the RRC layer performs functions includingsession management and mobility management.

One cell managed by an eNB is set to one of the bandwidths of 1.25, 2.5,5, 10, 15, and 20 MHz, and provides a DL or UL transmission service to aplurality of UEs in the bandwidth. Different cells may be configured toprovide different bandwidths.

DL transport channels used to deliver data from the E-UTRAN to UEsinclude a Broadcast Channel (BCH) carrying system information, a PagingChannel (PCH) carrying a paging message, and a Shared Channel (SCH)carrying user traffic or a control message. DL multicast traffic orcontrol messages or DL broadcast traffic or control messages may betransmitted on a DL SCH or a separately defined DL Multicast Channel(MCH). UL transport channels used to deliver data from a UE to theE-UTRAN include a Random Access Channel (RACH) carrying an initialcontrol message and a UL SCH carrying user traffic or a control message.Logical channels that are defined above transport channels and mapped tothe transport channels include a Broadcast Control Channel (BCCH), aPaging Control Channel (PCCH), a Common Control Channel (CCCH), aMulticast Control Channel (MCCH), a Multicast Traffic Channel (MTCH),etc.

FIG. 3 illustrates physical channels and a general method fortransmitting signals on the physical channels in the 3GPP system.

Referring to FIG. 3, when a UE is powered on or enters a new cell, theUE performs initial cell search (S301). The initial cell search involvesacquisition of synchronization to an eNB. Specifically, the UEsynchronizes its timing to the eNB and acquires a cell Identifier (ID)and other information by receiving a Primary Synchronization Channel(P-SCH) and a Secondary Synchronization Channel (S-SCH) from the eNB.Then the UE may acquire information broadcast in the cell by receiving aPhysical Broadcast Channel (PBCH) from the eNB. During the initial cellsearch, the UE may monitor a DL channel state by receiving a DownLinkReference Signal (DL RS).

After the initial cell search, the UE may acquire detailed systeminformation by receiving a Physical Downlink Control Channel (PDCCH) andreceiving a Physical Downlink Shared Channel (PDSCH) based oninformation included in the PDCCH (S302).

If the UE initially accesses the eNB or has no radio resources forsignal transmission to the eNB, the UE may perform a random accessprocedure with the eNB (S303 to S306). In the random access procedure,the UE may transmit a predetermined sequence as a preamble on a PhysicalRandom Access Channel (PRACH) (S303 and S305) and may receive a responsemessage to the preamble on a PDCCH and a PDSCH associated with the PDCCH(S304 and S306). In the case of a contention-based RACH, the UE mayadditionally perform a contention resolution procedure.

After the above procedure, the UE may receive a PDCCH and/or a PDSCHfrom the eNB (S307) and transmit a Physical Uplink Shared Channel(PUSCH) and/or a Physical Uplink Control Channel (PUCCH) to the eNB(S308), which is a general DL and UL signal transmission procedure.Particularly, the UE receives Downlink Control Information (DCI) on aPDCCH. Herein, the DCI includes control information such as resourceallocation information for the UE. Different DCI formats are definedaccording to different usages of DCI.

Control information that the UE transmits to the eNB on the UL orreceives from the eNB on the DL includes a DL/UL ACKnowledgment/NegativeACKnowledgment (ACK/NACK) signal, a Channel Quality Indicator (CQI), aPrecoding Matrix Index (PMI), a Rank Indicator (RI), etc. In the 3GPPLTE system, the UE may transmit control information such as a CQI, aPMI, an RI, etc. on a PUSCH and/or a PUCCH.

FIG. 4 illustrates a structure of a radio frame used in the LTE system.

Referring to FIG. 4, a radio frame is 10 ms (327200×T_(s)) long anddivided into 10 equal-sized subframes. Each subframe is 1 ms long andfurther divided into two slots. Each time slot is 0.5 ms (15360×T_(s))long. Herein, T_(s) represents a sampling time and T_(s)=1/(15kHz×2048)=3.2552×10⁻⁸ (about 33 ns). A slot includes a plurality ofOrthogonal Frequency Division Multiplexing (OFDM) symbols or SC-FDMAsymbols in the time domain by a plurality of Resource Blocks (RBs) inthe frequency domain. In the LTE system, one RB includes 12 subcarriersby 7 (or 6) OFDM symbols. A unit time during which data is transmittedis defined as a Transmission Time Interval (TTI). The TTI may be definedin units of one or more subframes. The above-described radio framestructure is purely exemplary and thus the number of subframes in aradio frame, the number of slots in a subframe, or the number of OFDMsymbols in a slot may vary.

FIG. 5 illustrates exemplary control channels included in a controlregion of a subframe in a DL radio frame.

Referring to FIG. 5, a subframe includes 14 OFDM symbols. The first oneto three 01-DM symbols of a subframe are used for a control region andthe other 13 to 11 OFDM symbols are used for a data region according toa subframe configuration. In FIG. 5, reference characters R1 to R4denote RSs or pilot signals for antenna 0 to antenna 3. RSs areallocated in a predetermined pattern in a subframe irrespective of thecontrol region and the data region. A control channel is allocated tonon-RS resources in the control region and a traffic channel is alsoallocated to non-RS resources in the data region. Control channelsallocated to the control region include a Physical Control FormatIndicator Channel (PCFICH), a Physical Hybrid-ARQ Indicator Channel(PHICH), a Physical Downlink Control Channel (PDCCH), etc.

The PCFICH is a physical control format indicator channel carryinginformation about the number of OFDM symbols used for PDCCHs in eachsubframe. The PCFICH is located in the first OFDM symbol of a subframeand configured with priority over the PHICH and the PDCCH. The PCFICHincludes 4 Resource Element Groups (REGs), each REG being distributed tothe control region based on a cell Identity (ID). One REG includes 4Resource Elements (REs). An RE is a minimum physical resource defined byone subcarrier by one 01-DM symbol. The PCFICH is set to 1 to 3 or 2 to4 according to a bandwidth. The PCFICH is modulated in Quadrature PhaseShift Keying (QPSK).

The PHICH is a physical Hybrid-Automatic Repeat and request (HARQ)indicator channel carrying an HARQ ACK/NACK for a UL transmission. Thatis, the PHICH is a channel that delivers DL ACK/NACK information for ULHARQ. The PHICH includes one REG and is scrambled cell-specifically. AnACK/NACK is indicated in one bit and modulated in Binary Phase ShiftKeying (BPSK). The modulated ACK/NACK is spread with a Spreading Factor(SF) of 2 or 4. A plurality of PHICHs mapped to the same resources forma PHICH group. The number of PHICHs multiplexed into a PHICH group isdetermined according to the number of spreading codes. A PHICH (group)is repeated three times to obtain diversity gain in the frequency domainand/or the time domain.

The PDCCH is a physical DL control channel allocated to the first n OFDMsymbols of a subframe. Herein, n is 1 or a larger integer indicated bythe PCFICH. The PDCCH occupies one or more CCEs. The PDCCH carriesresource allocation information about transport channels, PCH andDL-SCH, a UL scheduling grant, and HARQ information to each UE or UEgroup. The PCH and the DL-SCH are transmitted on a PDSCH. Therefore, aneNB and a UE transmit and receive data usually on the PDSCH, except forspecific control information or specific service data.

Information indicating one or more UEs to receive PDSCH data andinformation indicating how the UEs are supposed to receive and decodethe PDSCH data are delivered on a PDCCH. For example, on the assumptionthat the Cyclic Redundancy Check (CRC) of a specific PDCCH is masked byRadio Network Temporary Identity (RNTI) “A” and information about datatransmitted in radio resources (e.g. at a frequency position) “B” basedon transport format information (e.g. a transport block size, amodulation scheme, coding information, etc.) “C” is transmitted in aspecific subframe, a UE within a cell monitors, that is, blind-decodes aPDCCH using its RNTI information in a search space. If one or more UEshave RNTI “A”, these UEs receive the PDCCH and receive a PDSCH indicatedby “B” and “C” based on information of the received PDCCH.

FIG. 6 illustrates a structure of a UL subframe in the LTE system.

Referring to FIG. 6, a UL subframe may be divided into a control regionand a data region. A Physical Uplink Control Channel (PUCCH) includingUplink Control Information (UCI) is allocated to the control region anda Physical uplink Shared Channel (PUSCH) including user data isallocated to the data region. The middle of the subframe is allocated tothe PUSCH, while both sides of the data region in the frequency domainare allocated to the PUCCH. Control information transmitted on the PUCCHmay include an HARQ ACK/NACK, a CQI representing a downlink channelstate, an RI for Multiple Input Multiple Output (MIMO), a SchedulingRequest (SR) requesting UL resource allocation. A PUCCH for one UEoccupies one RB in each slot of a subframe. That is, the two RBsallocated to the PUCCH are frequency-hopped over the slot boundary ofthe subframe. Particularly, PUCCHs with m=0, m=1, and m=2 are allocatedto a subframe in FIG. 6.

Hereinafter, a MIMO system will be described. MIMO refers to a method ofusing multiple transmission antennas and multiple reception antennas toimprove data transmission/reception efficiency. Namely, a plurality ofantennas is used at a transmitting end or a receiving end of a wirelesscommunication system so that capacity can be increased and performancecan be improved. MIMO may also be referred to as ‘multi-antenna’ in thisdisclosure.

MIMO technology does not depend on a single antenna path in order toreceive a whole message. Instead, MIMO technology collects datafragments received via several antennas, merges the data fragments, andforms complete data. The use of MIMO technology can increase systemcoverage while improving data transfer rate within a cell area of aspecific size or guaranteeing a specific data transfer rate. MIMOtechnology can be widely used in mobile communication terminals andrelay nodes. MIMO technology can overcome the limitations of therestricted amount of transmission data of single antenna based mobilecommunication systems.

The configuration of a general MIMO communication system is shown inFIG. 7.

A transmitting end is equipped with N_(T) transmission (Tx) antennas anda receiving end is equipped with N_(R) reception (Rx) antennas. If aplurality of antennas is used both at the transmitting end and at thereceiving end, theoretical channel transmission capacity increasesunlike the case where only either the transmitting end or the receivingend uses a plurality of antennas. Increase in channel transmissioncapacity is proportional to the number of antennas, thereby improvingtransfer rate and frequency efficiency. If a maximum transfer rate usinga signal antenna is R_(o), a transfer rate using multiple antennas canbe theoretically increased by the product of the maximum transfer rateR_(o) by a rate increment R_(i). The rate increment R_(i) is representedby the following equation 1 where R_(i) is the smaller of N_(T) andN_(R).

R _(i)=min(N _(T) ,N _(R))  [Equation 1]

For example, in a MIMO communication system using four Tx antennas andfour Rx antennas, it is possible to theoretically acquire a transferrate four times that of a single antenna system. After theoreticalincrease in the capacity of the MIMO system was first demonstrated inthe mid-1990s, various techniques for substantially improving datatransfer rate have been under development. Several of these techniqueshave already been incorporated into a variety of wireless communicationstandards including, for example, 3rd generation mobile communicationand next-generation wireless local area networks.

Active research up to now related to MIMO technology has focused upon anumber of different aspects, including research into information theoryrelated to MIMO communication capacity calculation in various channelenvironments and in multiple access environments, research into wirelesschannel measurement and model derivation of MIMO systems, and researchinto space-time signal processing technologies for improvingtransmission reliability and transfer rate.

To describe a communication method in a MIMO system in detail, amathematical model thereof is given below. As shown in FIG. 7, it isassumed that N_(T) Tx antennas and N_(R) Rx antennas are present. In thecase of a transmission signal, a maximum number of transmittable piecesof information is N_(T) under the condition that N_(T) Tx antennas areused, so that transmission information can be represented by a vectorrepresented by the following equation 2:

s=[s ₁ ,s ₂ , . . . , s _(N) _(T) ]^(T)  [Equation 2]

Meanwhile, individual transmission information pieces s₁, s₂, . . . ,s_(N) _(T) may have different transmission powers. In this case, if theindividual transmission powers are denoted by P₁, P₂, . . . , P_(N) _(T), transmission information having adjusted transmission powers can berepresented by a vector shown in the following equation 3:

ŝ=[ŝ ₁ ,ŝ ₂ , . . . , ŝ _(N) _(T) ]^(T)=[P ₁ s ₁ ,P ₂ s ₂ , . . . , P_(N) _(T) s _(N) _(T) ]^(T)

The transmission power-controlled transmission information vector ŝ maybe expressed as follows, using a diagonal matrix P of a transmissionpower:

$\begin{matrix}{\hat{s} = {{\begin{bmatrix}P_{1} & \; & \; & 0 \\\; & P_{2} & \; & \; \\\; & \; & \ddots & \; \\0 & \; & \; & {\; P_{N_{T}}}\end{bmatrix}\begin{bmatrix}s_{1} \\s_{2} \\\vdots \\s_{N_{T}}\end{bmatrix}} = {Ps}}} & \left\lbrack {{Equation}\mspace{14mu} 4} \right\rbrack\end{matrix}$

N_(T) transmission signals x₁, x₂, . . . , x_(N) _(T) to be actuallytransmitted may be configured by multiplying the transmissionpower-controlled information vector ŝ by a weight matrix W. In thiscase, the weight matrix is adapted to properly distribute transmissioninformation to individual antennas according to transmission channelsituations. The transmission signals x₁, x₂, . . . , x_(N) _(T) can berepresented by the following Equation 5 using a vector X. In Equation 5,W_(ij) is a weight between the i-th Tx antenna and the j-th informationand W is a weight matrix, which may also be referred to as a precodingmatrix.

$\begin{matrix}{x = {\left\lbrack \begin{matrix}x_{1} \\x_{2} \\\vdots \\x_{i} \\\vdots \\x_{N_{T}}\end{matrix} \right\rbrack = {{\begin{bmatrix}w_{11} & w_{12} & \ldots & w_{1\; N_{T}} \\w_{21} & w_{22} & \ldots & w_{2N_{T}} \\\vdots & \; & \ddots & \; \\w_{i\; 1} & w_{i\; 2} & \ldots & w_{{iN}_{T}} \\\vdots & \; & \ddots & \; \\w_{N_{T}1} & w_{N_{T}2} & \ldots & w_{N_{T}N_{T}}\end{bmatrix}\begin{bmatrix}{\hat{S}}_{1} \\{\hat{S}}_{2} \\\vdots \\{\hat{S}}_{j} \\\vdots \\{\hat{S}}_{N_{T}}\end{bmatrix}} = {{W\hat{s}} = {WPs}}}}} & \left\lbrack {{Equation}\mspace{14mu} 5} \right\rbrack\end{matrix}$

Generally, the physical meaning of a rank of a channel matrix may be amaximum number of different pieces of information that can betransmitted in a given channel. Accordingly, since the rank of thechannel matrix is defined as the smaller of the number of rows orcolumns, which are independent of each other, the rank of the matrix isnot greater than the number of rows or columns A rank of a channelmatrix H, rank(H), is restricted as follows.

rank(H)≤min(N _(r) ,N _(R))  [Equation 6]

Each unit of different information transmitted using MIMO technology isdefined as a ‘transmission stream’ or simply ‘stream’. The ‘stream’ maybe referred to as a ‘layer’. The number of transmission streams is notgreater than a rank of a channel which is a maximum number of differentpieces of transmittable information. Accordingly, the channel matrix Hmay be indicted by the following Equation 7:

# of streams≤rank(H)≤min(N _(T) ,N _(R))  [Equation 7]

-   -   where ‘# of streams’ denotes the number of streams. It should be        noted that one stream may be transmitted through one or more        antennas.

There may be various methods of allowing one or more streams tocorrespond to multiple antennas. These methods may be described asfollows according to types of MIMO technology. The case where one streamis transmitted via multiple antennas may be called spatial diversity,and the case where multiple streams are transmitted via multipleantennas may be called spatial multiplexing. It is also possible toconfigure a hybrid of spatial diversity and spatial multiplexing.

Now, a description of a Channel status information (CSI) report isgiven. In the current LTE standard, a MIMO transmission scheme iscategorized into open-loop MIMO operated without CSI and closed-loopMIMO operated based on CSI. Especially, according to the closed-loopMIMO system, each of the eNB and the UE may be able to performbeamforming based on CSI to obtain a multiplexing gain of MIMO antennas.To obtain CSI from the UE, the eNB allocates a PUCCH or a PUSCH tocommand the UE to feedback CSI for a downlink signal.

CSI is divided into three types of information: a Rank Indicator (RI), aPrecoding Matrix Index (PMI), and a Channel Quality Indicator (CQI).First, RI is information on a channel rank as described above andindicates the number of streams that can be received via the sametime-frequency resource. Since RI is determined by long-term fading of achannel, it may be generally fed back at a cycle longer than that of PMIor CQI.

Second, PMI is a value reflecting a spatial characteristic of a channeland indicates a precoding matrix index of the eNB preferred by the UEbased on a metric of Signal-to-Interference plus Noise Ratio (SINR).Lastly, CQI is information indicating the strength of a channel andindicates a reception SINR obtainable when the eNB uses PMI.

An advanced system such as an LTE-A system considers additionalmulti-user diversity through multi-user MIMO (MU-MIMO). Due tointerference between UEs multiplexed in an antenna domain in MU-MIMO,the accuracy of CSI may significantly affect interference with othermultiplexed UEs as well as a UE that reports the CSI. Accordingly, moreaccurate CSI than in single-user MIMO (SU-MIMO) should be reported inMU-MIMO.

In this context, the LTE-A standard has determined to separately designa final PMI as a long-term and/or wideband PMI, W1, and a short-termand/or subband PMI, W2.

For example, a long-term covariance matrix of channels expressed asEquation 8 may be used for hierarchical codebook transformation thatconfigures one final PMI with W1 and W2.

W=norm(W1W2)

In Equation 8, W2 is a short-term PMI, which is a codeword of a codebookreflecting short-term channel information, W is a codeword of a finalcodebook, and norm(A) is a matrix obtained by normalizing each column ofmatrix A to 1.

Conventionally, the codewords W1 and W2 are given as Equation 9.

$\begin{matrix}{{{{W\; 1(i)} = \begin{bmatrix}X_{i} & 0 \\0 & X_{i}\end{bmatrix}},{{{where}\mspace{14mu} X_{i}\mspace{14mu} {is}\mspace{14mu} {{Nt}/2}\mspace{14mu} {by}\mspace{14mu} M\mspace{14mu} {{Matrix}.W}\; 2(j)} = \overset{}{\begin{bmatrix}e_{M}^{k} & e_{M}^{l} & \; & e_{M}^{m} \\\; & \; & \ldots & \; \\{\alpha_{j}e_{M}^{k}} & {\beta_{j}e_{M}^{l}} & \; & {\gamma_{j}e_{M}^{m}}\end{bmatrix}}}}\mspace{14mu} {\left( {{{if}\mspace{14mu} {rank}} = r} \right),{{{where}\mspace{14mu} 1} \leq k},l,{m \leq {M\mspace{14mu} {and}\mspace{14mu} k}},l,{m\mspace{14mu} {are}\mspace{14mu} {{integer}.}}}} & \left\lbrack {{Equation}\mspace{14mu} 9} \right\rbrack\end{matrix}$

Herein, N_(T) represents the number of transmission antennas, and Mrepresents the number of columns in a matrix Xi, indicating that thematrix Xi has a total of M candidate column vectors. e_(ji) ^(k), e_(ij)^(l) and e_(ij) ^(m) represent k^(th), l^(th), and m^(th) column vectorsof Xi, in which k^(th), l^(th), and m^(th) elements are 0 and theremaining elements are 1, respectively. α_(j), β_(j), and γ_(j) arecomplex values each having a unit norm, indicating that when k^(th),l^(th), and m^(th) column vectors of the matrix Xi are selected, phaserotation is applied to these column vectors, respectively. i is aninteger equal to or larger than 0, representing a PMI indicating W1, andj is an integer equal to or larger than 0, representing a PMI indicatingW2.

In Equation 9, the codewords are designed so as to reflect correlationcharacteristics between established channels, if cross-polarizedantennas are densely arranged, for example, the distance betweenadjacent antennas is equal to or less than half a signal wavelength. Thecross-polarized antennas may be divided into a horizontal antenna groupand a vertical antenna group and the two antenna groups are co-located,each having the property of a uniform linear array (ULA) antenna.

Therefore, the correlations between antennas in each group have the samelinear phase increment property and the correlation between the antennagroups is characterized by phase rotation. Since a codebook is quantizedvalues of channels, it is necessary to design a codebook reflectingchannel characteristics. For convenience of description, a rank-1codeword designed in the above manner may be given as Equation 10.

$\begin{matrix}{{W\; 1(i)*W\; 2(j)} = \begin{bmatrix}{X_{i}(k)} \\{\alpha_{j}{X_{i}(k)}}\end{bmatrix}} & \left\lbrack {{Equation}\mspace{14mu} 10} \right\rbrack\end{matrix}$

In Equation 10, a codeword is expressed as an N_(T)×1 vector where N_(T)is the number of Tx antennas and the codeword is composed of an uppervector X_(i)(k) and a lower vector α_(i)X_(i)(k) representing thecorrelation characteristics of the horizontal and vertical antennagroups, respectively. X_(i)(k) is expressed as a vector having thelinear phase increment property, reflecting the correlationcharacteristics between antennas in each antenna group. For example, adiscrete Fourier transform (DFT) matrix may be used for X_(i)(k).

As described before, CSI includes, but not limited to, a CQI, a PMI, andan RI, and all or a part of the CQI, the PMI, and the RI are transmittedaccording to the transmission mode of each UE in the LTE system.Periodic CSI transmission is referred to as periodic CSI reporting, andCSI transmission upon request of an eNB is referred to as aperiodic CSIreporting.

In aperiodic CSI reporting, an eNB transmits a request bit included inUL scheduling information to a UE. The UE then transmits CSI based onits transmission mode to the eNB on a PUSCH.

In periodic CSI reporting, a period, an offset in the period, and so onare signaled semi-statically to each UE on a subframe basis byhigher-layer signaling. The UE transmits CSI based on a transmissionmode at a predetermined period on a PUCCH to the eNB. If UL data alsoexists in a subframe carrying the CSI, the CSI is transmitted togetherwith the UL data on a PUSCH.

The eNB transmits, to each UE, transmission timing information suitablefor the UE in consideration of the channel state of the UE and thedistribution of UEs in a cell. The transmission timing information mayinclude a period, an offset, and so on required for CSI transmission,and may be transmitted to the UE by an RRC message.

A description will be given below of coordinated multi-point (CoMP)transmission/reception.

The introduction of a technique of increasing system performance byenabling cooperation between a plurality of cells is intended for abeyond LTE-A system. The technique is called CoMP. In CoMP, two or moreeNBs, access points (APs), or cells cooperate with each other forcommunication with a specific UE, in order to enable more reliablecommunication between the specific UE and the eNBs, APs, or cells. Inthe present invention, the terms eNB, AP, and cell are interchangeablyused in the same meaning.

Generally in a multi-cellular environment with a frequency reuse factorof 1, the performance of a UE at a cell edge and an average sectorthroughput may be reduced in view of inter-cell interference (ICI). Toreduce ICI, the legacy LTE system allows a UE located at a cell edge tohave an appropriate throughput performance in aninterference-constrained environment by adopting a simple passivetechnique such as factional frequency reuse (FER) based on UE-specificpower control. However, it may be more preferable to reduce ICI or reuseICI as a desired signal for a UE than to reduce per-cell frequencyresource use. To this end, a CoMP transmission scheme may be used.

FIG. 8 illustrates an example of carrying out CoPM. Referring to FIG. 8,a wireless communication system includes a plurality of BSs that performCoMP, BS1, BS2 and BS3, and a UE. The plurality of BSs that performCoMP, BS1, BS2 and BS3 may efficiently transmit data to the UE bycooperating with each other.

CoMP transmission schemes may be classified into CoMP-joint processing(CoMP-JP) called cooperative MIMO characterized by data sharing, andCoMP-coordinated scheduling/beamforming (CoMP-CS/CB).

In DL CoMP-JP, a UE may receive data simultaneously from BSs thatperform CoMP transmission, and combine the received signals, therebyincreasing reception performance (joint transmission (JT)). In addition,one of the BSs participating in the CoMP transmission may transmit datato the UE at a specific time point (dynamic point selection (DPS)). InCoMP-CS/CB, a UE may receive data instantaneously from one BS, that is,a serving BS by beamforming.

In UL CoMP-JP, a plurality of BSs may receive a PUSCH signal from a UEat the same time (joint reception (JR)). In contrast, in UL CoMP-CS/CB,only one BS may receive a PUSCH from a UE. Herein, cooperative cells (orBSs) may make a decision as to whether to use CoMP-CS/CB.

A UE adopting a CoMP transmission scheme, that is, a CoMP UE may feedback channel information (referred to as a CSI feedback) to a pluralityof BSs that perform the CoMP transmission scheme. A network schedulermay select an appropriate CoMP transmission scheme that increases atransmission rate from among CoMP-JP, CoMP-CS/CB, and DPS on the basisof the CSI feedback. For this purpose, the CoMP UE may follow aPUCCH-based periodic feedback transmission scheme by configuring CSIfeedbacks for the plurality of BSs performing the CoMP transmissionscheme. In this case, feedback configurations for the respective BSs maybe independent of each other. Accordingly, each operation of feedingback channel information with such an independent feedback configurationwill be referred to as a CSI process in the disclosure according to anembodiment of the present invention. One or more CSI processes may existin one serving cell.

FIG. 9 illustrates a DL CoMP operation.

In FIG. 9, a UE is located between two eNBs, eNB1 and eNB2. The two eNBs(i.e., eNB1 and eNB2) perform an appropriate CoMP operation such as JT,DCS, or CS/CB in order to cancel interference to the UE. The UEtransmits an appropriate CSI feedback to assist the eNBs in the CoMPoperation. Information transmitted by the CSI feedback may include PMIinformation and CQI information for each eNB, and may further includechannel information between the two eNBs (e.g., phase offset informationbetween channels of the two eNBs), for JT.

In FIG. 9, although the UE transmits a CSI feedback signal to itsserving cell, eNB1, the UE may transmit a CSI feedback signal to eNB2 orboth of the eNBs, when needed. Further, while a basic unit participatingin CoMP is shown in FIG. 9 as an eNB, the present invention is alsoapplicable to CoMP between transmission points (TPs) controlled by asingle eNB.

That is, to enable the network to perform CoMP scheduling, the UE shouldfeedback DL CSI for a neighbor eNB/TP participating in CoMP as well asDL CSI for the serving eNB/TP. For this purpose, the UE transmitsfeedbacks for a plurality of CSI processes which reflect various datatransmitting eNBs/TPs and various interference environments.

Therefore, an interference measurement resource (IMR) is used to measureinterference for use in calculating CoMP CSI in the LTE system. Aplurality of IMRs may be configured for one UE, and the UE has anindependent configuration for each of the IMRs. That is, a period, anoffset, and a resource configuration are independently configured foreach IMR, and the eNB may signal the independent configuration for eachIMR to the UE by higher-layer signaling (RRC signaling or the like).

In addition, the LTE system uses a CSI-RS to measure a desired channel,which is required for calculating CoMP CSI. A plurality of CSI-RSs maybe configured for one UE, and each of the CSI-RSs may have anindependent configuration. That is, a period, an offset, a resourceconfiguration, power control (PC), and the number of antenna ports maybe configured independently for each CSI-RS, and the eNB signalsinformation related to the CSI-RS to the UE by higher-layer signaling(RRC signaling or the like).

One CSI process may be configured for a UE by associating one CSI-RS forsignal measurement with one IMR for interference measurement from amonga plurality of CSI-RSs and a plurality of IMRs configured for the UE.The UE feeds back CSI derived from different CSI processes withindependent periods and subframe offsets to the network (e.g., an eNB).

That is, each CSI process has an independent CSI feedback configuration.The eNB may indicate a per-CSI process CSI-RS resource, IMR, and CSIfeedback configuration to the UE by higher-layer signaling such as RRCsignaling or the like. For example, it is assumed that three CSIprocesses as listed in Table 1 are configured for the UE.

TABLE 1 CSI Process Signal Measurement Resource (SMR) IMR CSI Process 0CSI-RS 0 IMR 0 CSI Process 1 CSI-RS 1 IMR 1 CSI Process 2 CSI-RS 0 IMR 2

In [Table 1], CSI-RS 0 and CSI-RS 1 are respectively a CSI-RS receivedfrom the serving eNB of the UE, eNB1, and a CSI-RS received from aneighbor eNB participating in CoMP, eNB2. It is assumed that IMRs areconfigured for the CSI processes listed Table 1, as illustrated in Table2.

TABLE 2 IMR eNB 1 eNB 2 IMR 0 Muting Data transmission IMR 1 Datatransmission Muting IMR 2 Muting Muting

It is configured that in IMR 0, eNB1 is mute, eNB2 transmits data, andthe UE measures interference from the other eNBs except for eNB1.Similarly, it is configured that in IMR 1, eNB2 is mute, eNB1 transmitsdata, and the UE measures interference from the other eNBs except foreNB2. Further, it is configured that in IMR 2, both of eNB1 and eNB2 aremute, and the UE measures interference from the other eNBs except foreNB1 and eNB2.

Therefore, as noted from Table 1 and Table 2, CSI of CSI process 0represents best RI, PMI, and CQI information, in the case where the UEreceives data from eNB1. CSI of CSI process 1 represents best RI, PMI,and CQI information, in the case where the UE receives data from eNB2.CSI of CSI process 2 represents best RI, PMI, and CQI information, inthe case where the UE receives data from eNB1 without any interferencefrom eNB2.

The introduction of an active antenna system (AAS) is a recent activeresearch area for future-generation mobile communication. As eachantenna is configured as an active antenna having an active circuit inthe AAS, the AAS is a technology that may be applied more efficientlyfor interference reduction or beamforming by changing an antenna patternadaptively according to a situation.

When the AAS is implemented two-dimensionally, that is, atwo-dimensional AAS (2D-AAS) is realized, it is possible to change atransmission beam more actively according to the location of a receiverby more efficiently controlling the main lobe of an antennathree-dimensionally in an antenna pattern.

FIG. 10 illustrates an implementation example of the 2D-AAS.Particularly, FIG. 10 is based on the assumption of a co-polarizedantenna array in which each antenna element has the same polarization.Referring to FIG. 10, as antennas are arranged vertically andhorizontally in the 2D-AAS, it is expected that the 2D-AAS will bedeployed as a large-scale antenna system.

In a full dimension-MIMO (FD-MIMO) system to which the 2D-AAS isapplied, an eNB may configure a plurality of CSI-RS resources in one CSIprocess for a UE. A CSI process refers to an operation of feeding backchannel information with an independent feedback configuration.

In this case, the UE assumes the CSI-RS resources configured in thesingle CSI process to be one huge CSI-RS resource by aggregating theCSI-RS resources, calculates CSI from the aggregate, and feeds back thecalculated CSI. For example, the eNB configures three 4-port CSI-RSresources in one CSI process for the UE, and the UE assumes one 12-portCSI-RS resource by aggregating the three 4-port CSI-RS resources. The UEcalculates CSI by using a 12-port PMI from the CSI-RS resource, andfeeds back the CSI. This reporting mode is referred to as class A CSIreporting in the LTE-A system.

Or, the UE selects one of the CSI-RS resources, assuming that each ofthe CSI-RS resources is an independent channel, and calculates andreports CSI based on the selected resource. That is, the UE selects aCSI-RS having a strong channel from among the eight CSI-RSs, calculatesCSI based on the selected CSI-RS, and reports the CSI to the eNB. The UEadditionally reports the selected CSI-RS to the eNB by a CSI-RS resourceindicator (CRI). For example, if the channel of a first CSI-RScorresponding to T(0) is strongest, the UE reports a CRI set to 0(CRI=0) to the eNB. This reporting mode is referred to as class B CSIreporting in the LTE-A system.

To represent the above features more effectively, the followingparameters may be defined for a CSI process in class B. K represents thenumber of CSI-RS resources in one CSI process, and N_(k) represents thenumber of CSI-RS ports in a k^(th) CSI-RS resource.

<Hybrid CSI>

Meanwhile, hybrid CSI has recently been introduced for the 3GPPstandardization, in order to further improve the FD-MIMO system. Withreference to the drawings, hybrid CSI will be described in detail.

FIG. 11 is an exemplary view illustrating the concept of hybrid CSI.

Referring to FIG. 11, two enhanced MIMO (eMIMO) types are defined in asingle CSI process. CSI is produced for each eMIMO type. CSI of thefirst eMIMO type is fed back over a longer term, or carries wideband(WB) CSI, compared to CSI of the second eMIMO type. That is, the eNBconfigures a single CSI process with the first eMIMO type and the secondeMIMO type for the UE (i.e., the eNB transmits CSI process informationto the UE by higher-layer signaling), and changes beamforming applied toa CSI-RS of the second eMIMO type, using CSI of the first eMIMO typereceived from the UE. The UE then reports CSI based on the CSI-RS of thesecond eMIMO type to the eNB.

In FIG. 11, CSI of the first eMIMO type and CSI of the second eMIMO typeare defined respectively as first CSI (i.e., a first CRI, a first RI,first W1, first W2, and a first CQI) and second CSI (i.e., a second CRI,a second RI, second W1, second W2, and a second CQI).

Table 3 below illustrates exemplary operation mechanisms for hybrid CSI,reflected in the 3GPP standardization. Specifically, in hybrid CSIreporting mechanism 1, whether to transmit an RI is for further study(FFS), and W1 is reported in the first CSI. K, which represents thenumber of CSI-RSs defined in one class B CSI process, is 1. In thesecond CSI, therefore, a CRI is not reported, an RI, W2, and a CQI arereported, and whether to report W1 depends on a class B PMIconfiguration of the second eMIMO type, indicated by RRC signaling(i.e., if the PMI configuration is 0, W1 is reported, and if the PMIconfiguration is 1, W1 is not reported).

TABLE 3 First First First First Second Second Second Second Second TypeCRI RI W1 W2/CQI CRI RI W1 W2 CQI Mechanism 1 A + B N.A FFS ◯ X X ◯Depending on ◯ ◯ w/K = 1 (Class A) (K = 1) PMI config Mechanism 2 B + B◯ X X X X ◯ ◯ ◯ ◯ w/K = 1 (K > 1) (K = 1)

Now, a description will be given of the present invention with referenceto Table 3 and FIG. 11. It is to be noted that Table 3 and FIG. 11 aremerely exemplary for the convenience of description, and the spirit ofthe present invention is also applicable to any modification to thehybrid CSI reporting mechanisms.

<Methods for Periodic Feedback of Hybrid CSI>

First, a new reporting type for a PUCCH hybrid CSI feedback, and afeedback period and offset for the new reporting type will be described.

For periodic CSI reporting on a PUCCH, reporting types, and a feedbackperiod and offset for each reporting type should be newly introduced. Asdescribed before, as two eMIMO types are defined in a single CSIprocess, CSI of each eMIMO type should be reported periodically.

First of all, the period of first eMIMO-type CSI may be determined to bea multiple of a longest period of second eMIMO-type CSI. The longestperiod of CSI is the longest of the reporting periods of a CRI, an RI, aPMI, and a CQI. With transmission of a CRI, the longest period of thesecond eMIMO type-CSI is the period of the CRI, whereas without the CRI,the longest period of the second eMIMO-type CSI is the period of an RI.Obviously, if the RI is not transmitted, the longest period of thesecond eMIMO-type CSI is determined to be the period of a CQI. Or, theperiod of the first eMIMO-type CSI may always be set to a multiple ofthe period of the second eMIMO-type RI. Or, the period of the firsteMIMO-type CSI may always be set to a multiple of the period of thesecond eMIMO-type CQI.

The multiple may be set to a value, including 1, which makes the periodof the first eMIMO-type CSI equal to the longest period of the secondeMIMO-type CSI. Or the multiple may be fixed only to 1 so that theperiod of the first eMIMO-type CSI is always equal to the longest periodof the second eMIMO-type CSI. Or the multiple may be set not to 1 but toany value equal to or larger than 2 so that the period of the firsteMIMO-type CSI is always longer than the longest period of the secondeMIMO-type CSI.

An offset for the first eMIMO-type CSI, that is, a subframe offsetlinked to the period for the first eMIMO-type CSI may be limited to avalue equal to an offset for the longest CSI of the second eMIMO type.In this case, the first CSI and the second CSI collide with each otherin a specific subframe. One of methods of avoiding the collision is toreport the first CSI and the second CSI in the event of collision, anddefine a new reporting type for the reporting. Another method is toprioritize the first CSI and the second CSI such that the first CSIalways has a higher priority than the second CSI, and thus report thefirst CSI, while the second CSI is dropped. Similarly in the event ofCSI collision between different two CSI processes, it is defined thatthe first CSI has a higher priority than the second CSI or legacy CSI ofa legacy CSI process. Or, to overcome the collision by eNBimplementation, different offset configurations may be enabled bysignaling an offset of the first eMIMO-type CSI separately from anoffset of the second eMIMO-type CSI.

Further, the offset of the first eMIMO-type CSI is calculated based onthe offset of the longest period of the second eMIMO-type CSI (e.g., theperiod of the RI), and the offset of the first eMIMO-type CSI is limitedto a multiple of 5 which is the minimum of N_(pd) values. As aconsequence, a reporting time of the first eMIMO-type CSI is apart froma reporting time of the second eMIMO-type CSI by at least 5 ms, so thatthe UE secures a long time enough to calculate each of the firsteMIMO-type CSI and the second eMIMO-type CSI, that is, 5 ms or longer.Or for the same purpose, the constraint may be imposed that the offsetof the first eMIMO-type CSI is set to at least 5 ms. Or the constraintmay be imposed that the offset of the first eMIMO-type CSI is set to atleast N ms, where N may be RRC-signaled to the UE by the eNB.

If the offset of the first eMIMO-type CSI is limited to a value equal tothe offset of the longest period of the second eMIMO-type CSI, the firsteMIMO-type CSI should have a period at least longer than the longestperiod of the second eMIMO-type CSI. This is because if the same periodis set for the two pieces of CSI, the two pieces of CSI always collidewith each other, thereby resulting in dropping of the longest CSI of thesecond eMIMO type all the time. To avert the problem, the period of thefirst CSI should be set to a value equal to or longer than a double ofthe longest period of the second eMIMO-type CSI.

The following description is given on the assumption that offsets fromthe same reference subframe are set for the two pieces of CSI (i.e., thefirst CSI and the CSI with the longest period in the second CSI).Therefore, the same offset for the two pieces of CSI means that if theperiod of one CSI is set to N, and the period of the other CSI is set toN*K, the CSI having the period of N always collides with the CSI havingthe period of N*K. If an offset for one CSI is configured from asubframe in which the other CSI is reported, not from the same referencesubframe as for the offset for the other CSI, this implies that if theoffset is 0, the two pieces of CSI have the same offset. Therefore, ifan offset for one CSI is configured from a subframe in which the otherCSI is reported, this amounts to substantially setting the offset to 0.

Besides the scheme of reporting one of the first CSI and the second CSI,and dropping the other CSI in the event of collision between the firstCSI and the second CSI, a scheme of reporting the first CSI and thesecond CSI together may also be considered, as described before.However, since a payload size may increase above the maximum capacity ofPUCCH format 2, PUCCH format 2 may be switched to PUCCH format 3. Forexample, if the first W1 (or the first W1 and the first RI) collideswith the second CRI (or the second RI), the UE uses PUCCH format 3 toreport the first W1 and the second CRI (or the second RI) together.

Or the period and offset of the first eMIMO-type CSI may be signaledseparately from those of the second eMIMO-type CSI, and the first CSImay be configured to be transmitted over a long term, compared to thesecond CSI, by eNB implementation. That is, the period and offset of thefirst eMIMO-type CSI and the period and offset of the first eMIMO-typeCSI (the second eMIMO-type CSI?) are signaled separately, and a periodand an offset are configured for each eMIMO type in a legacy period andoffset configuration method. Notably, in order to prevent the offset ofthe first eMIMO-type CSI from being equal to the offset of the longestCSI of the second eMIMO type, the same offset may be excluded fromavailable values to be signaled.

If a plurality of pieces of CSI of the first eMIMO type are defined(i.e., if the UE reports two or more of the first CRI, RI, PMI, andCQI), the plurality of pieces of CSI of the first eMIMO type may belimited to the same period. That is, the first CRI, RI, PMI, and CQIhave equal periods. However, different offsets may be configured for thefirst CRI, RI, PMI, and CQI in view of the limited capacity of thePUCCH, and as a result, the first CRI, RI, PMI, and CQI may be reportedseparately in different subframes.

When a new PUCCH reporting type is introduced, the reporting type mayinclude only the first CSI, or both of the first CSI and the second CSI.In the former case, a bit concatenation order identical or similar to alegacy bit concatenation order may be determined for the first CSI. Thatis, the most significant bit (MSB) is allocated in the order of CRI, RI,W1, W2, and CQI. For example, first CRI+first W1 or first CRI+first W2may be introduced as a new reporting type, and the CRI and W1 arebit-concatenated with the CRI occupying the MSB and W1 occupying theleast significant bit (LSB). Herein, the UE does not report the firstRI, and always assumes the first RI to be 1.

In the latter case, that is, if the first CSI and the second CSI formone reporting type, a new bit concatenation order may be applied,instead of the legacy bit concatenation order for CSI. For example, ifthe first W1 (or the first W2) and the second CRI or the second RI arereported together, the first W1 (or the first W2) may first be allocatedto the MSB. This operation is intended to increase reliability byapplying more robust coding to the first PMI/CQI because the firstPMI/CQI is more important than the second CRI/RI and has thelong-term/WB property in hybrid CSI reporting. Then, the generalizationmay be made that the first CSI (i.e., the first CRI, RI, W1, W2, or CQI)is allocated to the MSB with priority over the second CSI (i.e., thesecond CRI, RI, W1, W2, or CQI).

The afore-described method of configuring a period and offset for firstCSI will be described in the context of the foregoing mechanisms, by wayof example. A method proposed in each mechanism is also applicable inthe same manner to other mechanisms. The eNB configures one of themechanisms for the UE by RRC signaling, and the UE reports CSI accordingto the configured mechanism. As a mechanism configuration is defined ineach CSI process, it is possible to report CSI in a different mechanismfor each CSI process. However, the first eMIMO type and the second eMIMOtype may share one resource for an IMR, thereby minimizing resourcewaste.

—Mechanism 1

As described above, the first W1 has a period set to a multiple of theperiod of the second RI, and an offset equal to the offset of the secondRI. The multiple may be indicated by reusing signaling used for settingthe period of the legacy CRI. That is, conventionally, a CRI has aperiod set to a multiple of the period of an RI, and an offset equal tothe offset of the RI, and signaling used for configuring the CRI periodis reused in the same manner for configuring the period of the first W1.Or a reserved state of the signaling used in configuring the period ofthe legacy CRI may be newly defined so as to set the period of the firstW1 to a multiple of the period of the second RI. This may be extended tothe generalization that the signaling used for configuring the period ofthe legacy CRI is reused to configure the period of the first CSI. Orthe reserved state of the signaling used for configuring the period ofthe legacy CRI may be redefined.

For reference, tables used in configuring the period and offset of thelegacy CRI are cited as Table 4 and Table 5. Table 4 lists CRI periodswhich are configured when an RI is transmitted, and Table 5 lists CRIperiods and offsets which are configured when an RI is not transmitted.If a CRI and an RI are not transmitted in the second eMIMO type, theperiod and offset of the first CSI may be determined based on the periodand offset (i.e., N_(pd)) of a CQI by reusing signaling illustrated inTable 5.

TABLE 4 I_(CRI) Value of M_(CRI) 0 1 1 2 2 4 3 8 4 16 5 32 6 64 7 128 7< I_(CRI) ≤ 1023 Reserved

TABLE 5 Value of I_(CRI) M_(CRI) Value of N_(OFFSET, CRI)  0 ≤ I_(CRI) ≤160 1 −I_(CRI) 161 ≤ I_(CRI) ≤ 321 2 −(I_(CRI) − 161) 322 ≤ I_(CRI) ≤482 4 −(I_(CRI) − 322) 483 ≤ I_(CRI) ≤ 643 8 −(I_(CRI) − 483) 644 ≤I_(CRI) ≤ 804 16 −(I_(CRI) − 644) 805 ≤ I_(CRI) ≤ 965 32 −(I_(CRI) −805)  966 ≤ I_(CRI) ≤ 1023 Reserved

In this case, collision may occur between a report of the first W1 and areport of the second RI. To avoid the collision, a new reporting typemay be defined to report the first W1 and the second RI together.However, since the new reporting type suffers from low resolution of W1due to the limited capacity of PUCCH format 2 (i.e., due tolow-precision caused by sub-sampling of a W1 codebook), the newreporting type may not be favorable. Although it is obvious that thetransmission capacity problem may be solved by using PUCCH format 3, itmay be regulated that the first W1 is reported, while the second RI isdropped. Or the period of the first W1 is set to a multiple of theperiod of the second RI, whereas the offset of the first W1 is signaledseparately from the offset of the second eMIMO type (i.e., the offset ofthe second RI), thereby enabling configuration of different offsets forthe first W1 and the second eMIMO type (i.e., the second RI). Or it maybe configured that the period and offset of the first W1 are signaledseparately from those of the second CSI.

If the first RI is additionally reported, it may be configured that thefirst RI is transmitted together with the first W1. Or although thefirst RI is reported with the same period as that of the first W1,different offsets are assigned to the first RI and the first W1 so thatthe first RI and the first W1 are transmitted in different subframes.Herein, the RI should be transmitted earlier than W1 by setting theoffset of the RI to be smaller than the offset of W1. Or the first RIand the first W1 are transmitted in different subframes by setting theperiod of the first RI to a multiple of the period of the first W1, andassigning different offsets to the first RI and the first W1.

In the case where although the first RI is reported with the same periodas that of the first W1, different offsets are assigned to the first RIand the first W1 so that the first RI and the first W1 are transmittedin different subframes, the eNB configures an offset only for one of thefirst RI and the first W1 for the UE, and an offset for the other isalways fixed to a value obtained by adding (or subtracting) apredetermined value to (or from) the signaled offset. For example, ifthe offset of the first RI is signaled to the UE, the offset of thefirst W1 is always set to a value calculated by adding 1 to the offsetof the first RI. This method may reduce signaling overhead. Likewise,this offset configuration method is also applicable to other mechanisms,when the first CSI includes multiple types of CSI (RI, PMI, CQI, CRI,etc.).

If the first RI is not reported, the UE and the eNB always assume thatthe rank of the first W1 is 1. Or the eNB indicates the first RI for usein calculating the first W1 to the UE by RRC signaling or the like, andthe UE calculates the first W1 based on the first RI. Or while the RI isnot reported for the convenience of period configuration in PUCCH CSItransmission, the RI may also be reported in PUSCH CSI transmission.

—Mechanism 2

In mechanism 2, if K>1 in the first eMIMO type, the first CSI mayinclude only the first CRI, or include the first RI, PMI, and CQI. IfK=1 in the first eMIMO type, various implementation examples areavailable, and the UE reports the first W1, the first W2, or the firstRI and the first PMI/CQI.

When the first CRI is reported, the first CRI has a period set to amultiple of the period of the second RI, and an offset equal to theoffset of the second RI. The multiple may be indicated by reusing thesignaling used for configuring a period of the legacy CRI. That is,conventionally, a CRI has a period set to a multiple of the period of anRI, and an offset equal to the offset of the RI, and signaling used forconfiguring the CRI period is reused in the same manner for configuringthe period of the first CRI. Or a reserved state of the signaling usedin configuring the period of the legacy CRI may be newly defined so asto set the period of the first CRI to a multiple of the period of thesecond RI. If a CRI and an RI are not transmitted in the second eMIMOtype, the period and offset of the first CRI may be determined based onthe period and offset (i.e., N_(pd)) of a CQI by reusing signalingillustrated in Table 5.

In this case, when the first CRI is reported, a report of the first CRIalways collides with a report of the second RI. To avoid the collision,the UE defines and reports the first CRI and the second RI together inone reporting type (or in this case, it may be regulated that the firstCRI is reported, while the second RI is dropped). Or while the period ofthe first CRI is set to a multiple of the period of the second RI, theoffset of the first CRI may be signaled separately from the offset ofthe second eMIMO type (i.e., the offset of the second RI), therebyenabling configuration of different offsets. Or the period and offset ofthe first CRI are signaled and configured separately from those of thesecond CSI.

When the first RI, PMI, or CQI is reported, the UE configures the periodand offset of the first CSI by receiving signaling separately from theperiod and offset of the second CSI from the eNB. Herein, the period andoffset of the first CSI are configured in the same manner as the periodand offset of CSI for a legacy non-hybrid CSI-RS. Or the first RI, PMI,and CQI have the same period, which is set to a multiple of the periodof the second RI. As described before, the eNB may indicate the multipleto the UE by reusing the signaling used for configuring the period ofthe legacy CRI. However, different offsets may be assigned to the firstRI, PMI, and CQI in consideration of the limited capacity of a PUCCH,and as a result, the first RI, PMI, and CQI may be reported separatelyin different subframes.

The method of configuring the period and offset of first W1(particularly, the method described in mechanism 1) as proposed in thepresent disclosure may be used in the same manner for a method ofconfiguring the period and offset of first CSI (e.g., first W2) otherthan the first W1. For example, if the first W2 is transmitted inmechanism 2, the proposed method of configuring the period and offset offirst W1 may be used as a method of configuring the period and offset offirst W2.

If the first RI is additionally reported, it may be configured that thefirst RI is transmitted together with the first W1. Or although thefirst RI is reported with the same period as that of the first W1, adifferent offset is assigned to the first RI so that the first RI andthe first W1 are transmitted in different subframes. Herein, the RIshould be transmitted earlier than W1 by setting the offset of the RI tobe smaller than the offset of W1. Or the first RI and the first W1 aretransmitted in different subframes by setting the period of the first RIto a multiple of the period of the first W1, and assigning differentoffsets to the first RI and the first W1.

<Solution to Collision of Hybrid CSI>

The following proposal is about a method of prioritizing CSI. In theevent of collision between CSI, if the CSI has equal priorities, the CSIis prioritized by using CSI process indexes and component carrier (CC)indexes in a conventional manner.

Compared to the second CSI, the first CSI is defined as long-term CSI,and affects configuration of a second eMIMO-type CSI-RS anddetermination of the second CSI. Therefore, since upon occurrence ofcollision, the UE reports CSI with a high priority, while dropping CSIwith a low priority, the first CSI should have a higher priority thanthe second CSI. The prioritization is applied when collision occursbetween the first CSI and the second CSI in the same CSI process, andalso when collision occurs between the first CSI and the second CSI indifferent CSI processes/CCs.

In another method, different prioritizations may be applied to collisionbetween the first CSI and the second CSI in the same CSI process andcollision between the first CSI and the second CSI in different CSIprocesses/CCs.

First of all, when collision occurs between the first CSI and the secondCSI in the same CSI process, the first CSI has a higher priority thanthe second CSI. In this case, since too much priority may be given tothe first CSI, the first CSI and the second CSI may be prioritized in alegacy prioritization rule. That is, the CSI is prioritized in the orderof CRI>RI>PMI (or CQI). As a consequence, the second CRI has a higherpriority than the first RI. However, in the event of collision betweenCSI with the same priority such as the first CRI and the second CRI, thefirst CSI has a higher priority than the second CSI.

Upon occurrence of collision between CSI in different CSI processes/CCs,the legacy prioritization rule is applied irrespective of the first CSI,the second CSI, or the legacy non-hybrid CSI. Or upon occurrence ofcollision between CSI in different CSI processes/CCs, the rule ofprioritizing first CSI and second CSI as proposed by the presentinvention is applied only to collision between the first CSI and thesecond CSI, while the same prioritization rule is applied to all of theother cases (e.g., collision between the first CSI and the legacynon-hybrid CSI (or the legacy CSI), collision between the second CSI andthe legacy CSI, collision between the second CSI and the second CSI, andcollision between the first CSI and the first CSI) irrespective of thefirst CSI, the second CSI, or the legacy non-hybrid CSI.

Or a highest priority is assigned only to part of the first CSI, whichis considered to be important, while the remaining first CSI isprioritized in the same manner as the legacy CSI. For example,considering that the first eMIMO type mainly functions to indicate thedirection of a channel, the PMI (W1 or W2), the PMI and RI, or the CRIin the first CSI is considered to be important. Therefore, a higherpriority is assigned to the first PMI (the first W1 or the first W2),the first PMI and the first RI, or the first CRI than the legacy CSI andthe second CSI, while an existing priority is applied to the remainingCSI (e.g., the first CQI).

Or the priority of the first CSI is set to be equal to that of thelegacy CRI, and the second CSI is prioritized in the same manner as thelegacy CSI (i.e., the priority of the legacy CSI is the priority of CSIfor a non-hybrid CSI-RS). The legacy CRI is reported with a highestpriority in the legacy CSI, and the priority of the first CSI isconsidered to be equal to that of the CRI. For example, upon occurrenceof collision between the first W1 and the second RI, the first W1 isreported, while the RI is dropped.

Or upon occurrence of collision between the first CSI and the secondCSI, the legacy prioritization is applied. However, only in the case ofequal priorities, the first CSI may be reported, while the legacy CSImay be dropped. For example, upon occurrence of collision between W1 ofthe first CSI and the legacy second CRI, the second CRI is firstreported according to the legacy prioritization, whereas upon occurrenceof collision between the CRI of the first CSI and the CRI of the secondCSI, the first CRI is reported.

Meanwhile, in the event of collision between the first CSI and thelegacy non-hybrid CSI, priorities should be defined between the firstCSI and the legacy non-hybrid CSI. In view of the long-term nature ofthe first CSI as described before, the first CSI preferably has a higherpriority than the legacy CSI. For example, upon occurrence of collisionbetween the first W1 and the legacy CRI, the first W1 is reported, whilethe legacy CRI is dropped.

Or the highest priority (i.e., a priority higher than that of the legacyCSI) is assigned only to part of the first CSI, which is considered tobe important, while the remaining first CSI is prioritized in the samemanner as the legacy CSI.

Or the priority of the first CSI is set to be equal to the priority ofthe legacy CRI. Or the priority equal to the priority of the legacy CRIis assigned only to part of the first CSI, which is considered to beimportant, while the remaining first CSI is prioritized in the samemanner as the legacy CSI.

Or upon occurrence of collision between the first CSI and the legacyCSI, the legacy prioritization is applied. However, only if the firstCSI and the legacy CSI have equal priorities, the first CSI may bereported, while the legacy CSI may be dropped. For example, while uponoccurrence of collision between W1 of the first CSI and the CRI of thelegacy CSI, the CRI is first reported according to the legacyprioritization, upon occurrence of collision between the CRI of thefirst CSI and the CRI of the legacy CSI, the first CRI is reported.

However, the legacy prioritization rule may still be applied between thesecond CSI and the non-hybrid CSI. Or it may be configured that thepriority of the second CSI is always higher (or lower) than that of thenon-hybrid CSI.

Additionally, upon occurrence of collision between the first CSI, thelegacy prioritization may still be applied. For example, upon occurrenceof collision between the first CRI and the first CQI, the first CRI isreported according to the legacy prioritization. Specifically, uponoccurrence of collision between the first CSI of CSI process 1 and thefirst CSI of CSI process 2, the CSI has equal priorities. Thus, the CSIof a CSI process with a lower CSI process index or the CSI of a CC witha lower CC index is reported.

When the afore-described new reporting type is used, that is, the firstCSI and the second CSI are reported in one subframe through onereporting type, the reporting type is prioritized to have the priorityof the first CSI. That is, all reporting types including the first CSIfollow the priority of the first CSI. Or the priority of a correspondingreporting type is determined to be the higher between the priorities ofthe first CSI and the second CSI.

Meanwhile, a new reporting type for the first CSI may be defined suchthat the first CSI has a higher priority than the second CSI, in theevent of collision between the first CSI and the second CSI. Forexample, when the first W1 and the first RI are reported together, areporting type for the first W1 and the first RI is defined as type 5′separately from the legacy reporting type of W1 and an RI, type 5. Type5′ is reported with a higher priority than the remaining reporting typescorresponding to the second CSI or the legacy CSI (or type 5′ has apriority equal to the priorities of legacy types 7, 8, 9 and 10 in whicha CRI is reported). Likewise, when the first W1 is reported alone, areporting type for the first W1 is defined as type 2a′, separately fromthe legacy reporting type of W1, type 2a, and a higher priority isassigned to type 2a′. Similarly, when the first RI is reported alone, areporting type for the first RI is defined as type 3′, separately fromthe legacy reporting type of an RI, type 3, and a higher priority isassigned to type 3′. In this way, reporting of the first CSI may bedefined as type x′, referring to a legacy type, and type x′ has a higherpriority than the legacy types. The priority relationship between thelegacy types x may be applied to priorities between types x′. Forexample, since upon occurrence of collision between type 5 and type 2a,type 5 has priority over type 2a, this priority relationship is stillapplied, and as a result, type 5a′ has priority over type 2a′ uponoccurrence of collision between type 5′ and type 2a′. Or type x′ has apriority equal to the priorities of types 7, 8, 9, and 10 including aCRI among the legacy types.

<PUCCH Reporting Mode Configuring Method>

According to a conventional method, a wideband (WB)/subband (SB)reporting mode is determined by CQI reportPeriodicProclD defined in aCSI process, and a PMI reporting mode is determined by pmi-ri-Report ofCQI reportBothProc defined in the CSI process. As a result, a total offour PUCCH reporting modes are defined according to combinations ofWB/SB and the presence and absence of a PMI.

For hybrid CSI reporting, the WB/SB reporting mode of the second CSI isdetermined by CQI reportPeriodicProclD, while the WB reporting mode isalways determined for the first CSI irrespective of this signaling. Thisis because it is preferred to fix the reporting mode of the first CSI tothe WB reporting mode in view of the nature of the first CSI that thefirst CSI provides long-term/WB channel information, compared to thesecond CSI.

Or for hybrid CSI reporting, a common reporting mode for the first CSIand the second CSI may be determined by CQI reportPeriodicProclD. Thatis, if the WB reporting mode is configured by CQI reportPeriodicProclD,both the first CSI and the second CSI are reported in the WB reportingmode, and if the SB reporting mode is configured by CQIreportPeriodicProclD, both the first CSI and the second CSI are reportedin the SB reporting mode.

In a TDD system, an eNB may measure information about the directions andstrengths of total channels by SRS reception. When the eNB transmits a(beamformed) CSI-RS to a UE on the basis of the information, the UEreports an RI and a CQI on the basis of the CSI-RS. Considering thisoperation, the TDD system does not require the hybrid CSI-RS scheme asmuch as the TDD system. This is because while information about thedirections of total channels is obtained through the first eMIMO type,and then a beamformed CSI-RS is transmitted in a corresponding directionfor a certain time (i.e., until the next first CSI is reported) throughthe second eMIMO type in the hybrid CSI-RS scheme, an SRS takes over therole of the first CSI in the TDD system. Therefore, with only the FDDsystem targeted in hybrid CSI, the UE preferably calculates CSI,expecting or assuming that pmi-ri-Report is always enabled.

Obviously, if channel estimation is not carried out successfully from anSRS or much reciprocity is not available, hybrid CSI reporting may beadopted even in the TDD system. In this case, the UE may feed back thefirst RI/PMI so that the eNB may compensate a channel estimated by anSRS with the first RI/PMI, and may not report the second RI/PMI bydisabling or not configuring pmi-ri-Report. Therefore, pmi-ri-Report isapplied only to the second eMIMO type. Whether to report a PMI/RI of thefirst eMIMO type is determined according to a hybrid CSI mechanism. Thatis, RRC signaling of pmi-ri-Report is restricted to the second eMIMOtype. If pmi-ri-Report is configured by RRC signaling, the PMI and RI ofthe second eMIMO type are reported, and otherwise, the PMI and RI of thesecond eMIMO type are not reported.

Meanwhile, in the case where the first eMIMO type is configured as classA and the second eMIMO type is configured as class B in one CSI process,if PUCCH feedback mode 2-1 is configured for the CSI process, sub-mode 2does not exist for class A. Thus, CSI is fed back by applying sub-mode 2only to class B, and applying sub-mode 1 to class A. Additionally, ifPUCCH feedback mode 2-1 is configured, the UE transmits a report with aPTI always set to 0 in the first eMIMO type. That is, the UE reports WBCSI. This serves the original purpose of hybrid CSI to providelong-term/WB information by a CSI feedback of the first eMIMO type, andreporting short-term/SB information by a CSI feedback of the secondeMIMO type.

<Hybrid CSI Calculation Method>

Now, a description will be given of a hybrid CSI calculation method,particularly a method of calculating second CSI in the absence ofrecently reported first CSI.

According to a legacy operation, when a UE calculates CSI, a CRI affectscalculation of the other CSI, that is, an RI, a PMI, and a CQI, the RIaffects calculation of the PMI and the CQI, and the PMI affectscalculation of the CQI. That is, there is a hierarchical structurebetween CSI, and higher-layer CSI (e.g., CRI) affects calculation oflower-layer CSI (e.g., RI, PMI, and CQI). As a consequence, the UEcalculates lower-layer CSI on the assumption of a specific value ofhigher-layer CSI. If the higher-layer CSI is not reported as is the casewith collision-incurred dropping, the UE calculates the lower-layer CSIon the assumption of a specific value of the higher-layer CSI (e.g., thelowest of indexes or values of CSI).

For the hybrid CSI-RS, the same scheme is applied between the first CSI(i.e., first RI, PMI, CQI, and CRI), and between the second CSI.However, whether this hierarchical structure exists between the firstCSI and the second CSI, and whether one piece of CSI affects calculationof the other pieces of CSI may vary according to how hybrid CSI isoperated, that is, hybrid CSI mechanisms. Additionally, although uponreceipt of first CSI, the eNB changes a CSI-RS configuration of thesecond eMIMO type (e.g., a beam configuration applied to a beamformedCSI-RS), thereby eventually affecting second CSI, this operation istransparent to the UE and thus does not affect a CSI calculationoperation of the UE. Hereinbelow, a hierarchical structure between CSI,and factors affecting CSI calculation will be described in the contextof a CSI calculation operation of a UE.

The afore-described mechanism 1 may be considered as an exemplaryoperation method of hybrid CSI reporting in which first CSI does notaffect calculation of second CSI (or vice versa). In mechanism 1, the UEreports a first PMI, and the eNB performs beamforming for a CSI-RS ofthe second eMIMO type by using the first PMI. Subsequently, the UEcalculates second CSI by using the CSI-RS of the second eMIMO type.Herein, the UE calculates each of the first CSI and the second CSIaccording to a legacy CSI calculation method.

The afore-described mechanism 1 may be considered as an exemplaryoperation method of hybrid CSI reporting in which first CSI affectscalculation of second CSI (or vice versa). In mechanism 1, the UEreports a first RI and a first PMI to the eNB, and the eNB performsbeamforming for a CSI-RS of the second eMIMO type by using the first RIand the first PMI. Subsequently, the UE calculates second CSI by usingthe CSI-RS of the second eMIMO type. Herein, in the absence of arecently reported second RI when the UE calculates a second PMI/CQI, theUE calculates the second PMI/CQI based on a latest reported first RI.Similarly, in the absence of a first W1 when the UE calculates a secondW2/CQI, the UE calculates the second W2/CQI based on the latest reportedfirst W1.

Additionally, when the first eMIMO type and the second eMIMO type aredivided into two CSI processes, the eNB may indicate linkage between thetwo CSI processes to the UE, and when calculating CSI of the secondeMIMO type, the UE may use CSI of the first eMIMO type. For example, thelinkage may be indicated by setting the index of the CSI process of thefirst eMIMO type in the CSI process of the second eMIMO type.Specifically, the second PMI, the second CQI, and the second RI may becalculated based on the latest reported first RI or first CRI.

One of methods of establishing a hierarchical structure between firstCSI and second CSI is to determine the range of a second CQI by a firstCQI (or to report a CQI offset based on the first CQI in a second CQI).That is, a modulation and coding scheme (MCS) table to be used inreporting the second CQI may be determined by the first CQI.

Meanwhile, when a first RI is additionally reported, the first RI may beconfigured to be transmitted together with first W1. Since the payloadsize of a class A codebook is significantly increased to about 10 bits,the first RI and the first W1 are preferably reported in PUCCH format 3.By extending this operation, it is proposed that all CSI of the firsteMIMO type is transmitted at one time in PUCCH format 3. If CSI of thefirst eMIMO type includes an RI, a CQI, and a PMI, or a CQI, an RI, aCQI, and a PMI, the CSI of the first eMIMO type may be transmitted atone time in PUCCH format 3. Further, if all CSI of the first eMIMO typeis transmitted at one time in PUCCH format 3, the CSI report in PUCCHformat 3 has a higher priority than a CSI report in PUCCH format 2 inthe event of collision between the CSI reports. Or this may begeneralized into the statement that upon occurrence of collision betweena reporting type in which CSI is transmitted in PUCCH format 3 and areporting type in which CSI is transmitted in PUCCH format 2, PUCCHformat 3 is preferably reported with priority.

Or upon occurrence of collision between first CSI reported in PUCCHformat 3 and second CSI reported in PUCCH format, transmission of thesecond CSI together with the first CSI in PUCCH format 3 is preferred todropping one of the first CSI and the second CSI and reporting the otherCSI. However, only when PUCCH format 3 has a capacity enough to carryboth of the first CSI and the second CSI, PUCCH format 3 carries both ofthe first CSI and the second CSI, and otherwise, only the first CSI isreported in PUCCH format 3.

Additionally, as both of the first CSI and the second CSI aretransmitted together, the following new reporting types should bedefined. For example, a reporting type for reporting a first W1 and afirst RI together with various pieces of second CSI including a secondRI is defined in hybrid mechanism 1. Typically, if the first CSI has aperiod which is a multiple of the period of the second RI, and an offsetequal to the offset of the second RI (i.e., offset=0), the first CSIcollides with the second RI, and thus the following new reporting typesshould be transmitted in PUCCH format 3.

Type 11=first RI+first W1+second RI

Type 12=first RI+first W1+second RI+second W1

Type 13=first RI+first W1+second RI+second PTI

Type 13′=first RI+first W1(defined in case second RI is not defined, orsecond

CSI does not collide with first CSI).

Type 14=first RI+first W1+second CRI

Type 15=first RI+first W1+second CRI+second RI

Type 16=first RI+first W1+second CRI+second RI+second W1

Type 17=first RI+first W1+second CRI+second RI+second PTI

In hybrid CSI reporting mechanism 2, for example, the first CSI may bereported in two methods. In one of the methods, if it is defined thatfirst CSI=first CRI, a reporting type for reporting first CRI togetherwith various pieces of second CSI including a second RI is defined.Typically, if the first CSI has a period which is a multiple of theperiod of the second RI, and an offset equal to the offset of the secondRI (i.e., offset=0), the first CSI collides with the second RI, and thusthe following new reporting types should be transmitted in PUCCH format3. Or considering that the payload sizes of a CRI and an RI are notlarge, PUCCH format 2 may be used in this case, as is doneconventionally.

Type 18=first CRI+second RI

Type 19=first CRI+second RI+second W1

Type 20=first CRI+second RI+second PTI

Type 21=first CRI(defined in case second RI is not defined, or secondCSI does not collide with first CSI)

Types 18 to 21 are similar to the legacy types 7, 8, 9, and 10, exceptthat first CSI and second CSI are transmitted together in types 18 to21. As types 7 to 10 and types 18 to 21 are defined separately, higherpriorities may be assigned to types 18 to 21 than types 7 to 10.

In the other method, if it is defined that first CSI=first PMI, areporting type for reporting first PMI together with various pieces ofsecond CSI including a second RI is defined. Typically, if the first CSIhas a period which is a multiple of the period of the second RI, and anoffset equal to the offset of the second RI (i.e., offset=0), the firstCSI collides with the second RI, and thus the following new reportingtypes should be transmitted in PUCCH format 3.

Type 18=first PMI for specific first CSI-RS+first PMI for another firstCSI-RS+second RI

Type 19=first PMI for specific first CSI-RS+first PMI for another firstCSI-RS+second RI+second W1

Type 20=first PMI for specific first CSI-RS+first PMI for another firstCSI-RS+second RI+second PTI

Type 21=first PMI for specific first CSI-RS+first PMI for another firstCSI-RS (defined in case second RI is not defined, or second CSI does notcollide with first CSI)

In types 18 to 21, the specific first CSI-RS is the CSI-RS with thelower index between two CSI-RSs defined to be of the first eMIMO type,and another first CSI-RS is the CSI-RS with the higher index.

As various pieces of CSI are transmitted at one time through bitconcatenation in the above types, there is a need for defining a bitconcatenation order. First, MSBs are allocated to CSI in a describedorder in the above type definitions. For example, MSBs are allocated inthe order of the first RI, the first W1, and the second RI in type 11.

As described above, although the first PMI for the specific first CSI-RSand the first PMI for the other first CSI-RS may be transmitted at thesame time in PUCCH format 3, if PUCCH format 2 is used withoutintroduction of PUCCH format 3, only one first PMI is transmitted in onesubframe.

The period and offset of a first PMI report may be defined to be amultiple of the longest CSI period of the second CSI (e.g., the periodof the second RI), and the same or different offset, respectively. Thefirst PMI for the specific first CSI-RS and the first PMI for the otherfirst CSI-RS are transmitted alternately at the time of eachtransmission subframe for the first PMIs.

That is, if first PMI transmission timings are set as subframe # n,subframe # n+10, subframe # n+20, subframe # n+30 . . . , the first PMIfor the specific first CSI-RS is transmitted in subframe # n, subframe #n+20, subframe # n+40 . . . , and the first PMI for the other firstCSI-RS is transmitted in subframe # n+10, subframe # n+30, subframe #n+50 . . . .

Or instead of alternate reporting between the first PMI for the specificfirst CSI-RS and the first PMI for the other first CSI-RS, the first PMIfor the specific first CSI-RS and the first PMI for the other firstCSI-RS may be configured to be transmitted with the same period anddifferent offsets in different subframes

Or in order to ensure as much scheduling freedom as possible, differentperiods and different offsets may be configured for the first PMI forthe specific first CSI-RS and the first PMI for the other first CSI-RS.

Meanwhile, a CSI-RS may be selected by the first CRI, and reconfigureddynamically as a CSI-RS of the second eMIMO type. For example, in thecase where two 4-port CSI RS and 8-port CSI-RS exist in the first eMIMOtype, if the first CRI indicates the 4-port CSI-RS, the CSI-RS of thesecond eMIMO type is reconfigured as the 4-port CSI-RS, and if the firstCRI indicates the 8-port CSI-RS, the CSI-RS of the second eMIMO type isreconfigured as the 8-port CSI-RS. However, if the CSI-RS of the secondeMIMO type is reconfigured dynamically, additional signaling overhead isproduced. The overhead may be minimized by the following proposedmethods.

Proposal 1: the numbers of multiple ports of a first CSI-RS and a secondCSI-RS, or PCs of the first CSI-RS and the second CSI-RS (i.e., valuesthat determine power ratios between the CSI-RSs and a PDSCH) are limitedto the same value. That is, the UE does not expect that different portnumbers or PC values are configured for the first CSI-RS and the secondCSI-RS.

According to proposal 1, there is no need for reconfiguring the numberof ports or PC of the second CSI-RS irrespective of what CSI-RS isselected by a first CRI, thereby obviating the need for dynamicsignaling. Even though different resource and subframe configurationsare configured for the first and second CSI-RSs, a hybrid CSI operationis not affected, and thus the two parameters may be configured freely.However, proposal 1 suffers from lack of scheduling flexibility, andthus proposal 2 may be adopted to overcome the shortcoming.

Proposal 2: The number of ports of a second CSI-RS is set to the maximumnumber N of ports of a first CSI-RS. If a first CRI indicates a CSI-RSof fewer ports than N ports (e.g., K ports), the number of ports of thesecond CSI-RS is changed to K, and REs carrying a CSI-RS are determinedto be a subset of REs of the N-port second CSI-RS without additionalsignaling. For example, since 8 REs of a second CRI-RS with N of 8 and Kof 4 may be defined as two 4-port CSI-RSs, the CSI-RS with a lowersubcarrier index between the two 4-port CSI-RSs is determined. Inanother example, if N is 8 and K is 2, 4 of 8 REs of the second CSI-RSmay be defined as four 2-port CSI-RSs, and thus a CSI-RS with a lowsubcarrier index or a high subcarrier index may be determined from amongfour 2-port CSI-RSs.

Proposal 3: The UE overrides all or a part of the number of ports, Pc,or RE pattern of a second CSI-RS with the number of ports, Pc, or REpattern of a first CSI-RS selected by a CRI. However, since the secondCSI-RS should be transmitted with a shorter period than the firstCSI-RS, the subframe configuration of the second CSI-RS is notoverridden.

<Bit Concatenation Order for PUSCH CSI Feedback>

Meanwhile, as described before, if a first PMI for a specific firstCSI-RS and a first PMI for another first CSI-RS are reported together,the specific first CSI-RS may be the CSI-RS with the lower index betweentwo CSI-RSs defined to be of the first eMIMO type, and another firstCSI-RS may be the CSI-RS with the higher index. Further, the first PMIfor the specific first CSI-RS may first be allocated to MSBs,accompanied by allocation of the first PMI for the other first CSI-RS.Apparently, the opposite case is also possible.

<Method of Configuring P-CSI Subframe Period/Offset for P-CSI+AP-CSI>

Meanwhile, a scheme of reporting first CSI as periodic CSI (P-CSI) on aPUCCH and reporting second CSI as aperiodic CSI (AP-CSI) on a PUSCH areunder consideration for the current 3GPP standardization. (On thecontrary, a scheme of reporting second CSI as P-CSI on a PUCCH andreporting first CSI as AP-CSI on a PUSCH is also under consideration.Also in this case, the proposed methods of the present invention areapplicable in the same manner).

As such, since only the first CSI is configured as P-CSI, the eNB shouldconfigure the reporting period and offset of the first CSI for the UE.First of all, the reporting period and offset of the first CSI may beconfigured independently of those of the second CSI. Or the reportingperiod and offset of the first CSI may be configured to be referencesfor the reporting period and offset of the second CSI, and thus may beset relatively. A detailed description will be given below of each ofthe two cases.

A. When the reporting period and offset of the first CSI are configuredindependently of those of the second CSI, the legacy period and offsetconfiguration method may be used to configure the reporting period andoffset of the first CSI.

In the legacy configuration method, the period Npd and offsetN_(offset,CQI) of a minimum unit for reporting a CQI are configured, andthe periods and offsets of an RI, W1 and a CRI are calculated to bemultiples of Npd and offsets relative to N_(offset,CQI) by [Equation 11]to [Equation 14].

(10×n _(f) +└n _(s)/2┘−N _(OFFSET,CQI) −N _(OFFSET,RI))mod(N _(pd) ·M_(RI)=0  [Equation 11]—RI period

(10×n _(f) +└n _(s)/2┘−N _(OFFSET,CQI))mod(H′·N _(pd))=0  [Equation12]—Feedback period of W1 only

(10×n _(f) +└n _(s)/2┘−N _(OFFSET,CQI) −N _(OFFSET,CRI))mod(H·N _(pd) ·M_(CRI))  [Equation 13]—Feedback period of CRI only

(10×n _(f) +└n _(s)/2┘−N _(OFFSET,CQI) −N _(OFFSET,RI))mod(H·N ^(pd) ·M_(RI) ·M _(CRI))=0  [Equation 14]—Feedback period of CRI+RI

A case in which the legacy configuration method is used in configuringthe period and offset of first CSI will be described. In mechanism 1,since the first CSI includes an RI and W1, there is no periodic reportof a CQI and a CRI. However, to conform to the legacy configurationmethod, the eNB may configure a period and an offset for the CQI, andthen a period and an offset for the first CSI based on the period andoffset of the CQI. Although the period and offset of the CQI areconfigured, these values are used only to configure the period andoffset of the first CSI. In fact, the UE does not report a CQI at areporting time. As such, when the period and offset of the first CSI areconfigured in the legacy period and offset configuration method, the UEreports no CSI at the remaining configured (periodic) reporting timesexcept for the reporting time of the first CSI (i.e., the UE ignores theremaining (periodic) reporting times).

When the period and offset of the first CSI are configured in the legacyperiod and offset configuration method, the eNB configuresN_(offset,CQI), N_(offset,RI), Npd, and M_(RI) for the UE, and theperiod and offset of the first CSI are determined by [Equation 11]. Itis to be noted that considering that the CQI is not actually reported,N_(offset,RI) may be fixed to a specific value (e.g., 0), which maysimplify an RRC configuration.

When the period and offset of the first CSI are configured in the legacyW1 period configuration method, the eNB configures N_(offset,CQI), Npd,and H′ for the UE, and the period and offset of the first CSI aredetermined by [Equation 12]. However, since an existing H′ value islimited to a small value, it is preferable to redefine H′ so as toconfigure H′ to be a large value.

When the period and offset of the first CSI are configured in the legacyCRI period configuration method, the eNB configures N_(offset,CQI),N_(offset,CRI), Npd, and M_(CRI) for the UE, and the period and offsetof the first CSI are determined by [Equation 13]. Or when the period andoffset of the first CSI are configured in the legacy CRI periodconfiguration method, the eNB configures N_(offset,CQI), N_(offset,RI),Npd, M_(CRI), and MRI for the UE, and the period and offset of the firstCSI are determined by [Equation 14].

When the legacy RI/W1/CRI period configuration method is used asdescribed above, a related period and offset may be named independentlyfor the purpose of hybrid CSI. If the existing parameter names are stillused, confusion with a legacy CSI reporting configuration may make a UEoperation unclear. Therefore, the independent parameter naming isintended to prevent the ambiguity of the UE operation.

Meanwhile, in hybrid CSI reporting mechanism 1, the first CSI is dividedinto the first RI and the first W1. If the first RI and the first W1 arereported at one time in one PUCCH format, one of the proposed methodsmay be used. For example, if the legacy CRI period configuration methodis used, the eNB configures N_(offset,CQI), N_(offset,RI), Npd, M_(CRI),and M_(RI), but the UE reports only the first RI and the first W1without reporting the CQI, CRI, and W2. Therefore, the UE does notreport the CQI, CRI, and W2 at reporting times corresponding to the CQI,CRI, and W2. Instead, the UE reports the first RI and the first W1 onlyin subframes satisfying [Equation 4].

If the first RI and the first W1 should be reported at different timesdue to too large a payload size, the same period is shared between thefirst RI and the first W1, while reporting times may be separated bysignaling offsets separately for the first RI and the first W1 in theforegoing proposed method. For example, when the legacy CRI periodconfiguration method is used, the first RI may be reported in a subframesatisfying [Equation 15] below, and the first W1 may be reported in asubframe satisfying [Equation 16].

(10×n _(f)+└_(s)/2┘−N _(OFFSET,CQI) −N _(OFFSET RI_1))mod(H·N _(pd) ·M_(RI) ·M _(CRI))=0  [Equation 15]

(10×n _(f) +└n _(s)/2┘−N _(OFFSET,CQI) −N _(OFFSET RI_2))mod(mod(H·N_(pd) ·M _(RI) ·M _(CRI))=0  [Equation 16]

Also in this case, the UE does not report the CQI, CRI, and W2 atreporting periods of the CQI, the CRI, and W2.

Also in hybrid CSI reporting mechanism 2, the first CSI may be reportedin the same manner. Herein, the first CSI includes only the first CRI.

In the above proposal, the period and offset of the first CSI aredetermined relatively to the period and offset of the CQI. However, theperiod and offset of the first CSI may be configured without using theperiod and offset of the CQI by [Equation 17]. Notably,N_(offset,1st CSI) and N_(1st CSI) need to be defined as a new table in[Equation 17].

(10×n _(f) +└n _(s)/2┘−N _(OFFSET,1stCSI))mod(H′·N_(1stCSI))=0  [Equation 17]

B. When the period and offset of the first CSI are determined relativelyto the period and offset of the second CSI which are used as references,for example, the period of the first CSI may be determined to be amultiple of the period of the second CSI, and the offset of the firstCSI may be applied based on the offset of the second CSI. The offsets ofthe second CSI and the first CSI are signaled separately, and a finalsubframe offset for the first CSI is calculated by applying both of theoffsets of the first CSI and the second CSI.

In this case, while the period and offset of the second CSI aresignaled, these values are used only in configuring the period andoffset of the first CSI, and nothing should actually be reported at areporting time of the second CSI. This is because the second CSI isreported as AP-CSI to the eNB.

However, ambiguity exists in a UE operation as follows. It is not clearwhich one between a method of reporting both first CSI and second CSI asP-CSI (method B1) and a method of reporting first CSI as P-CSI andsecond CSI as AP-CSI (method B2) is to be performed by the UE. This isbecause the period and offset of the first CSI and the period and offsetof the second CSI are all configured in method B1 as well as method B2.If the eNB intends to adopt method B1, the UE should report nothing at areporting time of the second CSI, and if the eNB intends to adopt methodB2, the UE should report second CSI at a reporting time of the secondCSI. Therefore, the eNB should indicate which one between method B1 andmethod B2 is used for CSI reporting to the UE by RRC signaling or DCI.For this purpose, a 1-bit flag may be introduced.

On the contrary, if the reporting period and offset of the first CSI areconfigured independently of those of the second CSI, the ambiguity doesnot exist. This is because in method B1, the eNB will indicate all ofthe periods and offsets of the first CSI and the second CSI to the UE,and in method B2, the eNB will indicate only the period and offset ofthe first CSI to the UE. That is, this is because the two methods may bedistinguished from each other by signaling that configures a period andan offset.

Further, irrespective of whether the reporting period and offset of thefirst CSI are configured independently of those of the second CSI, ordetermined relatively, serving as references for the reporting periodand offset of the second CSI, the eNB should not configure method B1 andmethod B2 simultaneously for the UE. In other words, the UE does notexpect the eNB to configure method B1 and method B2 simultaneously.Since method B1 and method B2 contradict each other in the sense ofdisabling/enabling the periodic feedback of the second CSI, a UEoperation is ambiguous regarding simultaneous configuration of the twomethods.

On the other hand, if method B3 is defined as a “method of transmittingboth first CSI and second CSI as AP-CSI”, method B1 and method B3 may beconfigured simultaneously, and method B2 and method B3 may be configuredsimultaneously. If method B1 and method B3 are configuredsimultaneously, the first CSI and the second CSI are transmittedperiodically, and when needed (e.g., when one of the first CSI and thesecond CSI is dropped or ambiguity occurs between the eNB and the UEregarding an RRC reconfiguration period for P-CSI), the eNB may triggerboth of the first CSI and the second CSI or selectively trigger one ofthe first CSI and the second CSI, for AP-CSI reporting. If method B2 andmethod B3 are configured simultaneously, the first CSI is transmittedperiodically, and when needed, the eNB may trigger AP-CSI reporting ofthe second CSI. Or when the first CSI is dropped or ambiguity occursbetween the eNB and the UE regarding an RRC reconfiguration period forP-CSI, the eNB may trigger AP-CSI reporting of the first CSI. Or whenneeded, the eNB may trigger AP-CSI reporting for both of the first CSIand the second CSI.

Additionally, the following operation may be performed in hybrid CSIreporting mechanism 1. Also in hybrid CSI reporting mechanism 2, aperiod and an offset are configured for CSI of the first eMIMO type,that is, CSI(1) in the same manner.

-   -   The period of the first eMIMO-type CSI is an integer multiple of        the period of the second eMIMO-type RI, RI⁽²⁾.    -   The subframe offset of the first eMIMO-type CSI is determined by        the subframe offset of the second eMIMO-type RI, RI⁽²⁾.    -   For periodic CSI reporting in hybrid CSI reporting mechanism 1,        a subframe offset may be configured for CSI⁽¹⁾ (i.e., PMI⁽¹⁾ and        RI⁽¹⁾, or PMI⁽¹⁾ only). In this case, a legacy CRI period and        offset configuration may be reused for the period and offset of        CSI⁽¹⁾, as illustrated in Table 6 below.

TABLE 6 Value of I_(CSI) ⁽¹⁾ M_(CSI) ₍₁₎ Value of N_(OFFSET, CSI) ₍₁₎  0≤ I_(CSI) ⁽¹⁾ ≤ 160 1 −I_(CSI) ⁽¹⁾ 161 ≤ I_(CSI) ⁽¹⁾ ≤ 321 2 −(I_(CSI)⁽¹⁾ − 161) 322 ≤ I_(CSI) ⁽¹⁾ ≤ 482 4 −(I_(CSI) ⁽¹⁾ − 322) 483 ≤ I_(CSI)⁽¹⁾ ≤ 643 8 −(I_(CSI) ⁽¹⁾ − 483) 644 ≤ I_(CSI) ⁽¹⁾ ≤ 804 16 −(I_(CSI)⁽¹⁾ − 644) 805 ≤ I_(CSI) ⁽¹⁾ ≤ 965 32 −(I_(CSI) ⁽¹⁾ − 805)  966 ≤I_(CSI) ⁽¹⁾ ≤ 1023 Reserved

Further, in hybrid CSI reporting mechanism 1, the first eMIMO-type CSIhas a higher priority than the legacy CSI or the second eMIMO-type CSI.However, the following new porting types having a highest priorityshould be defined.

-   -   Type 5⁽¹⁾ for reporting RI⁽¹⁾ and W1 ⁽¹⁾    -   Type 2a⁽¹⁾ for reporting W1 ⁽¹⁾ only

It is to be noted that the second eMIMO-type CSI conforms to the legacyprioritization rule.

In hybrid CSI reporting mechanism 2, if the first CSI is defined asCRI⁽¹⁾ only, a new reporting type for reporting CRI⁽¹⁾, reporting type10⁽¹⁾ other than a legacy reporting type for reporting a CRI, that is,reporting type 10 is introduced, and has a higher priority than thelegacy reporting type. Or it is proposed that the newly definedreporting type has a priority equal to that of the legacy CRI.

<Relaxation in Hybrid CSI>

A relaxation method in hybrid CSI, that is, a method of determining whatCSI is to be calculated or updated on the part of a UE may be defined asfollows.

For an initial AP-CSI feedback trigger for a corresponding CSI process,which is received N subframes after reception of a first eMIMO-typeCSI-RS (hereinbelow, referred to as a first CSI-RS) from the eNB, the UEcalculates (updates) first CSI while not calculating (updating) secondCSI. For any other AP-CSI feedback trigger than the initial AP-CSIfeedback trigger, the second CSI is calculated (updated), while thefirst CSI is not calculated (updated). This operation will be describedwith reference to a drawing.

FIG. 12 illustrates an exemplary CSI relaxation method in hybrid CSIaccording to an embodiment of the present invention.

Referring to FIG. 12, the UE receives a first CSI-RS in subframe 1 andsubframe 11. As the UE receives an initial AP-CSI feedback trigger insubframe 2 after the reception of the first CSI-RS, the UE updates firstCSI without updating second CSI. As the UE receives a second AP-CSIfeedback trigger and a third AP-CSI feedback trigger in subframe 5 andsubframe 8, respectively after the reception of the first CSI-RS, the UEupdates the second CSI without updating the first CSI. As the UEreceives a new first CSI-RS in subframe 11 and then receives an initialAP-CSI feedback trigger in subframe 13, the UE updates the first CSIwithout updating the second CSI. As the UE receives a second AP-CSIfeedback trigger in subframe 14 after the reception of the first CSI-RS,the UE updates the second CSI without updating the first CSI.

While it has been defined above that for an initial AP-CSI feedbacktrigger (for a corresponding hybrid CSI process) received from the eNB,N subframes after reception of a new first CSI-RS, the UE calculates(updates) first CSI without calculating (updating) second CSI, for aninitial AP-CSI feedback trigger (for a corresponding hybrid CSI process)received from the eNB, with respect to a reception time of a new firstCSI-RS (i.e., including the reception time of the first CSI-RS), the UEmay calculate (update) first CSI without calculating (updating) secondCSI. As a consequence, if a trigger is configured in subframe 1, thefirst CSI is updated.

Further, while the above proposal is made based on a triggering time,the same thing may apply based on a CSI reference resource time insteadof a triggering time. Now, a description will be given of the presentinvention in the context of a reference resource.

For an initially defined CSI reference resource N subframes afterreception of a new first CSI-RS, the UE calculates (updates) first CSIwithout calculating (updating) second CSI. For any other CSI referenceresource than the initially defined CSI reference resource, the UEcalculates (updates) the second CSI without calculating (updating) thefirst CSI.

FIG. 13 illustrates another exemplary CSI relaxation method in hybridCSI according to an embodiment of the present invention.

Referring to FIG. 13, the UE receives a first CSI-RS in subframe 1 andsubframe 11. As an initial CSI reference resource is defined in subframe2 after the reception of the first CSI-RS, the UE updates first CSIwithout updating second CSI. As a second CSI reference resource and athird CSI reference resource are defined respectively in subframe 5 andsubframe 8 after the reception of the first CSI-RS, the UE updates thesecond CSI without updating the first CSI.

As an initial reference resource is defined in subframe 13 afterreception of a new first CSI-RS in subframe 11, the UE updates the firstCSI without updating the second CSI. As a second initial referenceresource is defined in subframe 14 after the reception of the new firstCSI-RS in subframe 11, the UE updates the second CSI without updatingthe first CSI.

Additionally, for an initially defined CSI reference resource withrespect to a reception time of a new first CSI-RS (i.e., including thereception time of the first CSI-RS), the UE may calculate (update) firstCSI without calculating (updating) second CSI. As a consequence, if areference resource is configured in subframe 1, the first CSI isupdated.

Meanwhile, if an AP-CSI reporting trigger for a corresponding CSIprocess is a trigger for first CSI in FIG. 12, the UE reports updatedfirst CSI (i.e., calculates and reports the first CSI) only for aninitial trigger from a subframe in which a first eMIMO-type CSI-RS isreceived until before receiving the next first eMIMO-type CSI-RS, andreports non-updated first CSI (i.e., reports the previously calculatedvalue without calculating the first CSI) for the subsequent triggers. Orthe UE performs the foregoing operation from a subframe carrying a firsteMIMO-type CSI-RS+N (e.g., N=1) until receiving the next firsteMIMO-type CSI-RS. Further, parts corresponding to the second CSI inFIGS. 12 and 13 are not necessary in view of the assumption of triggersfor first CSI.

For example, if a reference resource for a corresponding CSI process isa reference resource for first CSI in FIG. 13, the UE reports updatedfirst CSI (i.e., calculates and reports the first CSI) only for aninitial reference resource from a subframe in which a first eMIMO-typeCSI-RS is received until before receiving the next first eMIMO-typeCSI-RS, and reports non-updated first CSI (i.e., reports the previouslycalculated value without calculating the first CSI) for the subsequenttriggers. Or the UE performs the foregoing operation from a subframecarrying a first eMIMO-type CSI-RS+N (e.g., N=1) until receiving thenext first eMIMO-type CSI-RS.

Meanwhile, in FIG. 12, a first eMIMO-type CSI-RS is received in subframe1, and a new first eMIMO-type CSI-RS is received in subframe 11 due tothe 10-ms period of the CSI-RS. Notably, a triggering time is linked toa reference resource time, and the relationship between the triggeringtime and the reference resource time is determined according to thenumber of CSI processes configured for the UE. That is, if a single CSIprocess is configured for the UE, the triggering time coincides with thereference resource time, whereas if a plurality of CSI processes areconfigured, the reference resource time is defined to be the triggeringtime−1.

If the triggering time coincides with the reference resource time, uponreceipt of a trigger for first CSI, CSI is calculated by using thelatest CSI-RS from a time of receiving the latest CSI-RS until receivingthe next CSI-RS in FIG. 12. Therefore, if a plurality of triggers forfirst CSI are received from the reception time of the latest CSI-RSuntil before the reception of the next CSI-RS, the problem occurs thatCSI is calculated based on the same CSI-RS for all of the triggers forfirst CSI. In this context, the following is proposed to preventredundant calculation of the same CSI based on the same CSI-RS.

If CSI⁽¹⁾ for the same CSI-RS transmission of the 1^(st) eMIMO-type istriggered multiple times, the UE is required to update CSI⁽¹⁾ only oncefor the initial trigger.

If CSI⁽¹⁾ for the same CSI-RS transmission of the 1^(st) eMIMO-type istriggered multiple times, the UE is not required to update CSI⁽¹⁾ morethan once.

If the reference resource time is defined to be the triggering time−1,upon receipt of a trigger for first CSI, CSI is calculated by using thelatest CSI-RS from a time of receiving the latest CSI-RS+1 untilreceiving the next CSI-RS. Therefore, if a plurality of trigger forfirst CSI are received from the reception time of the latest CSI-RS+1until the reception of the next CSI-RS, the problem occurs that CSI iscalculated based on the same CSI-RS for all of the trigger for firstCSI. Also in this case, the following is proposed to prevent redundantcalculation of the same CSI based on the same CSI-RS.

If CSI⁽¹⁾ for the same CSI-RS transmission of the 1^(st) eMIMO-type istriggered multiple times, the UE is required to update CSI⁽¹⁾ only oncefor the initial trigger.

If CSI⁽¹⁾ for the same CSI-RS transmission of the 1^(st) eMIMO-type istriggered multiple times, the UE is not required to update CSI⁽¹⁾ morethan once.

Meanwhile, the first CSI carries only beam group information forbeamforming of a second eMIMO-type CSI-RS, and thus is calculated for afirst eMIMO-type CSI-RS transmitted over a long term. That is, the firstCSI is long-term information, and is intended to determine a beam group,separately from a CQI. Accordingly, there is no need for consideringinterference for the first CSI, unlike calculation of the legacy CSI.Instead, the first CSI should be calculated only based on a channelestimated from a first eMIMO-type CSI-RS. Further, considering that thefirst eMIMO-type CSI-RS is transmitted with a long period, calculationof an accuracy by interpolating a plurality of CSI-RSs is not helpful.Therefore, the UE calculates the first CSI only by using the lastestreceived first eMIMO-type CSI-RS.

In one method, after determining the dominant eigen vector of estimatedchannels, the UE selects a beam group having a highest correlation withthe dominant eigen vector, and reports the selected beam group. The UEeventually uses only the latest received first eMIMO-type CSI-RS withoutusing an IMR in calculating the first CSI. That is, For a firsteMIMO-type CSI-RS, a second eMIMO-type CSI-RS, and an IMR defined in ahybrid CSI process, the IMR is not used for calculating first CSI,whereas the IMR is used for calcualting second CSI. Or an IMR defined ina hybrid CSI process has no relation to a first eMIMO-type CSI-RS, andis linked to a second eMIMO-type CSI-RS. In a structure in which an IMRmay be defined separately per CSI-RS, an IMR (i.e., a linked IMR) shouldbe configured only for a second eMIMO-type CSI-RS, not a firsteMIMO-type CSI-RS.

Meanwhile, the UE uses a first eMIMO-type CSI-RS for calculating onlyfirst CSI, not second CSI during a time (e.g., 4 subframes) taken tocalculate the CSI using the CSI-RS from the reception time of the firsteMIMO-type CSI-RS (e.g., subframe # n). This operation will be describedwith reference to a drawing.

FIG. 14 illustrates an example of carrying out hybrid CSI reportingaccording to an embodiment of the present invention.

Referring to FIG. 14, only first CSI is calculated in subframes # n tosubframe # n+3 including a time when a first eMIMO-type CSI-RS isreceived, that is, subframe # n (the first eMIMO-type CSI-RS has aperiod of 10 ms). If any of subframe # n to subframe # n+3 is used forcalculating second CSI (for the same CSI process) due to an AP-CSIfeedback trigger for the second CSI, the UE does not calculate (update)the second CSI.

For example, when the eNB triggers second CSI reporting at the time ofsubframe # n−4, the UE completes calculation of second CSI in subframe #n−4 to subframe # n−1. In this case, since the subframes reserved forcalculating the first CSI, that is, subframe # n to subframe # n+3 donot overlap with the subframes for calculating the second CSI, thesecond CSI is calculated.

On the other hand, if the eNB triggers second CSI reporting in subframe# n−3, the UE completes calculation of second CSI in subframe # n−3 tosubframe # n. In this case, since subframe # n among the subframesreserved for calculating the first CSI, that is, subframe # n tosubframe # n+3 overlaps with the subframes for calculating the secondCSI, the second CSI is not calculated. Instead, non-updated second CSIis reported. That is, upon receipt of a first eMIMO-type CSI-RS insubframe # n, the UE reports non-updated CSI without calculating secondCSI for a second CSI reporting trigger received in subframe # n−3 tosubframe # n+3.

If the UE has received any trigger for first CSI before subframe # n,the UE may have calculated the first CSI for a previously received firsteMIMO-type CSI-RS in subframe # n−10 to subframe # n−7. Thus, the UE hasonly to report the calculated CSI. If the UE has received any triggerfor first CSI in subframe # n, the UE calculates CSI by using alreadysecured subframe # n to subframe # n+3, and reports the CSI in subframe# n+4. Further, if the UE has received any trigger for first CSI aftersubframe # n, the UE may have calculated CSI for the first eMIMO-typeCSI-RS received in subframe # n in subframe # n to subframe # n+3. Thus,the UE may report the calculated CSI.

In FIG. 14, regarding a triggering and reporting combination denoted bya dotted line, since CSI calculation overlaps with a CSI processing timereserved for first CSI, the corresponding CSI is not updated, andnon-updated CSI is reported. In contrast, regarding a triggering andreporting combination denoted by a solid line, since CSI calculationdoes not overlap with the CSI processing time reserved for the firstCSI, the corresponding CSI is updated and reported. In FIG. 14,triggering and reporting of second CSI are denoted by a dotted line or asold line. While triggering of first CSI is not shown in FIG. 14, the UEoperates in the same manner as described above.

The above proposal obviates calculation of second CSI during calculationof first CSI, thereby reducing implementation complexity for CSIcalculation at the UE.

FIG. 15 illustrates another example of carrying out hybrid CSI reportingaccording to an embodiment of the present invention. Compared to FIG.14, particularly, both of first CSI and second CSI are always triggered,and a reference resource time for the second CSI coincides with atriggering time in FIG. 15. A reference resource for the first CSI isfixed to a subframe carrying the latest received first eMIMO-typeCSI-RS. Since triggers for both of the first CSI and the second CSI arealways received at the same time, both of a UE operation for the firstCSI and a UE operation for the second CSI are performed in the proposaldescribed with reference to FIG. 14.

Referring to FIG. 15, only first CSI is calculated in subframe # n tosubframe # n+3 including a time when a first eMIMO-type CSI-RS isreceived, that is, subframe # n (the first eMIMO-type CSI-RS has aperiod of 10 ms). If any of subframe # n to subframe # n+3 is used forcalculating second CSI (for the same CSI process) due to an AP-CSIreporting trigger, the UE does not calculate or update the second CSI.

For example, if the eNB triggers CSI in subframe # n−4, the UE completescalculating second CSI in subframe # n−4 to subframe # n−1. The UEreports already calculated first CSI (i.e., non-updated first CSI). Inthis case, due to no overlap with the subframes reserved for calculationof the first CSI, that is, subframe # n to subframe # n+3, the secondCSI is calculated. In the case of no overlap with the subframes reservedfor calculation of the first CSI, that is, subframe # n to subframe #n+3, the second CSI is updated and reported, and otherwise, non-updatedsecond CSI is reported.

Meanwhile, if the eNB triggers CSI reporting for a corresponding CSIprocess at the time of subframe # n−3, the UE completes calculating CSIin subframe # n−3 to subframe # n. In this case, due to overlap withsubframe # n among subframes reserved for calculation of the first CSI,that is, subframe # n to subframe # n+3, non-updated second CSI andnon-updated first CSI are reported, without calculating the second CSI.That is, if the UE receives a first eMIMO-type CSI-RS in subframe # n,the UE reports non-updated CSI without calculating the second CSI/thefirst CSI for a CSI reporting trigger for a corresponding CSI process,received in subframe # n−3 to subframe # n−1. For a CSI reportingtrigger for the corresponding CSI process, received in subframe # n tosubframe # n+3, the UE reports non-updated second CSI withoutcalculating the second CSI, and reports first CSI calculated based on aCSI-RS received in subframe # n. For a CSI reporting trigger for thecorresponding CSI process, received from subframe # n+4, the UEcalculates and reports second CSI, and reports first CSI calculatedbased on a CSI-RS received in subframe # n.

FIGS. 12 to 15 are based on the assumption that all of CSI triggering,CSI reporting, and CSI-RS transmission relate to the same hybrid CSIprocess. Further, a second eMIMO-type CSI-RS is transmitted with ashorter period than a first eMIMO-type CSI-RS. For the convenience ofdescription, only the first eMIMO-type CSI-RS is shown without thesecond eMIMO-type CSI-RS in the drawings.

FIG. 16 is a block diagram of a BS and a UE which are applicable to anembodiment of the present invention.

Referring to FIG. 16, a wireless communication system includes a BS 110and a UE 120. The BS 110 includes a processor 112, a memory 114, and aradio frequency (RF) unit 116. The processor 112 may be configured toimplement the procedures and/or methods proposed in the presentinvention. The memory 114 is connected to the processor 112, and storesvarious types of information related to operations of the processor 112.The RF unit 116 is connected to the processor 112, and transmits and/orreceives wireless signals. The UE includes a processor 122, a memory124, and an RF unit 126. The processor 122 may be configured toimplement the procedures and/or methods proposed in the presentinvention. The memory 124 is connected to the processor 122, and storesvarious types of information related to operations of the processor 122.The RF unit 126 is connected to the processor 122, and transmits and/orreceives wireless signals. The BS 110 and/or the UE 120 may have asingle antenna or multiple antennas.

The embodiments of the present invention described above arecombinations of elements and features of the present invention. Theelements or features may be considered selective unless otherwisementioned. Each element or feature may be practiced without beingcombined with other elements or features. Further, an embodiment of thepresent invention may be constructed by combining parts of the elementsand/or features. Operation orders described in embodiments of thepresent invention may be rearranged. Some constructions of any oneembodiment may be included in another embodiment and may be replacedwith corresponding constructions of another embodiment. It is obvious tothose skilled in the art that claims that are not explicitly cited ineach other in the appended claims may be presented in combination as anembodiment of the present invention or included as a new claim by asubsequent amendment after the application is filed.

A specific operation described as performed by a BS may be performed byan upper node of the BS. Namely, it is apparent that, in a networkcomprised of a plurality of network nodes including a BS, variousoperations performed for communication with a UE may be performed by theBS, or network nodes other than the BS. The term ‘BS’ may be replacedwith the term ‘fixed station’, ‘Node B’, ‘evolved Node B (eNode B oreNB)’, ‘Access Point (AP)’, etc.

The embodiments of the present invention may be achieved by variousmeans, for example, hardware, firmware, software, or a combinationthereof. In a hardware configuration, the methods according to exemplaryembodiments of the present invention may be achieved by one or moreApplication Specific Integrated Circuits (ASICs), Digital SignalProcessors (DSPs), Digital Signal Processing Devices (DSPDs),Programmable Logic Devices (PLDs), Field Programmable Gate Arrays(FPGAs), processors, controllers, microcontrollers, microprocessors,etc.

In a firmware or software configuration, an embodiment of the presentinvention may be implemented in the form of a module, a procedure, afunction, etc. Software code may be stored in a memory unit and executedby a processor. The memory unit is located at the interior or exteriorof the processor and may transmit and receive data to and from theprocessor via various known means.

The memory unit is located inside or outside the processor, and maytransmit and receive data to and from the processor by various knownmeans.

Those skilled in the art will appreciate that the present invention maybe carried out in other specific ways than those set forth hereinwithout departing from the spirit and essential characteristics of thepresent invention. The above embodiments are therefore to be construedin all aspects as illustrative and not restrictive. The scope of theinvention should be determined by the appended claims and their legalequivalents, not by the above description, and all changes coming withinthe meaning and equivalency range of the appended claims are intended tobe embraced therein.

INDUSTRIAL APPLICABILITY

While the method and apparatus for feeding back hybrid CSI in amulti-antenna system have been described above in the context of a 3GPPLTE system, the method and apparatus are also applicable to variouswireless communication systems other than the 3GPP LTE system.

1. A method of transmitting channel status information (CSI)periodically to a base station by a user equipment (UE) in a wirelesscommunication system, the method comprising: receiving configurationrelated to one CSI process including a first enhanced multiple inputmultiple output (eMIMO) type and a second eMIMO type through a higherlayer; transmitting, to the base station, first CSI based on a firstchannel status information-reference signal (CSI-RS) associated with thefirst eMIMO type; and transmitting, to the base station, second CSIbased on a second beamformed CSI-RS associated with the second eMIMOtype, wherein a priority of the first CSI is equal to a priority of aCSI-RS resource indicator (CRI).
 2. The method according to claim 1,wherein the second beamformed CSI-RS is beamformed based on the firstCSI by the base station.
 3. The method according to claim 1, wherein ifthe first CSI including a precoding matrix index (PMI) collides with thesecond CSI including a rank indicator (RI), the second CSI is dropped.4. The method according to claim 1, wherein a reporting period of thefirst CSI is determined to be a multiple of a longest reporting periodof the second CSI.
 5. The method according to claim 4, wherein: when thesecond CSI includes a rank indicator (RI), the longest reporting periodof the second CSI is a reporting period of the RI included in the secondCSI, and when the second CSI does not include the RI, the longestreporting period of the second CSI is a reporting period of a channelquality indicator (CQI) included in the second CSI.
 6. A user equipment(UE) in a wireless communication system, the UE comprising: a memory;and at least one processor coupled to the memory and configured to:receive configuration related to one channel status information (CSI)process including a first enhanced multiple input multiple output(eMIMO) type and a second eMIMO type through a higher layer; transmit,to a base station, first CSI periodically based on a first channelstatus information-reference signal (CSI-RS) associated with the firsteMIMO type; and transmit, to the base station, second CSI periodicallybased on a second beamformed CSI-RS associated with the second eMIMOtype, wherein a priority of the first CSI is equal to a priority of aCSI-RS resource indicator (CRI).
 7. The UE according to claim 6, whereinthe second beamformed CSI-RS is beamformed based on the first CSI by thebase station.
 8. The UE according to claim 6, wherein if the first CSIincluding a precoding matrix index (PMI) collides with the second CSIincluding a rank indicator (RI), the second CSI is dropped.
 9. The UEaccording to claim 6, wherein a reporting period of the first CSI isdetermined to be a multiple of a longest reporting period of the secondCSI.
 10. The UE according to claim 9, wherein: when the second CSIincludes the RI a rank indicator (RI), the longest reporting period ofthe second CSI is a reporting period of the RI included in the secondCSI, and when the second CSI does not include the RI, the longestreporting period of the second CSI is a reporting period of a channelquality indicator (CQI) included in the second CSI.